Gangs:
The number of homicides committed by gang members declined in the late 1990s only to surge in 2001-2002 to more than 900 deaths a year, according to FBI records. Killings peaked at 1,287 in 1993 then dropped, falling to 698 in 1998 then increased to 940 in 2001. Street gangs now account for a record 93 percent of all homicides in which organized crime is the suspected cause.
Forty-six states have reported at least one gang-related killing since 1980. The only states not to do so are Delaware, New Hampshire, South Dakota and Vermont.

Hotbeds include: Los Angeles (330 deaths *), Chicago (97), Dallas, Baltimore, Miami, Newark, N.J., and Washington, D.C., New York (52 died in 2003), Long Island, Compton, CA (31), Long Beach (20), Santa Ana (17).
According to a Scripps Howard News Service study, Seventy-one percent of gangland killings occurred in California.
* Death counts are averages based on long term trend adjusted for current national rates.
Gang murders account for more than 2/3 of all murders in LA.
See LA Gangs.

Deaths are probably under-reported. "In many places, you don't want to tell your citizens that you have a problem with street gangs. If affects property values and depresses real estate sales." said Alex Alonso, a Los Angeles youth gang researcher.
Source: NBCActionNews.com

There are at least 21,500 gangs and more than 731,000 active gang members in the United States. Gangs conduct criminal activity in all 50 states and U.S. territories. Although most gang activity is concentrated in major urban areas, gangs also are proliferating in rural and suburban areas of the country
Source: Drugs and Gangs fast facts at the National Drug Intelligence Center of the DOJ.

Outlaw motorcycle gangs (OMGs):
Hells Angels, Outlaws, Pagans, and Bandidos

Hell's Angels are a NY motorcycle gang, usually racist whites. They actually have an organized drug dealing network that earns them money.

Mara Salvatrucha 13 (MS-13)
A gang organized in the mid-1980s in Los Angeles, known by its "MS-13" graffiti. Salvadorans, most of them former soldiers or guerrillas, formed it to protect themselves from rival Latino gangs. Membership has diversified to include people from Ecuador, Honduras, Mexico and, in smaller numbers, blacks. the most violent gang . The gang has carried out beheadings and grenade attacks in Central America and is known to hack their enemies with machetes in cities along the East Coast in the United States.

A Dec., 2004 attack on a bus in San Pedro Sula, Honduras, killed 28 people.

Mara Salvatrucha has more than 8,000 members in 27 states and the District of Columbia and also has more than 20,000 members in foreign countries, particularly El Salvador. The gang smuggles illicit drugs into the United States and transports and distributes drugs throughout the country. Members of Mara Salvatrucha often commit violent acts, principally against members of rival gangs. Traditionally, the gang was composed of loosely affiliated groups known as cliques; however, increased coordination of criminal activity among Mara Salvatrucha cliques in Los Angeles, Washington, D.C./Northern Virginia, and New York City may indicate that the gang is attempting to develop a national command structure.

In March 2005, 103 members of this gang were arrested in cities across the country.

Bloods and Crips:
Violent street gangs that formed in Los Angeles in the 1960s. The gangs are primarily African American in composition. Both gangs comprise loosely organized factions or sets that are typically turf- or neighborhood-oriented. Generally, gang sets are established by an entrepreneurial individual who runs the set and recruits members to distribute drugs, primarily cocaine. Bloods and Crips distribute drugs in California and in other states including North Carolina. Although Bloods and Crips sets are located throughout the United States, not all gangs that claim to be Bloods or Crips are affiliated with the Los Angeles-based Bloods or Crips. Bloods and Crips embrace the principles of individualism, loyalty to their own members, and violence against other gangs. Bloods and Crips harbor deep hatred toward each other, and each considers the other a rival. The two gangs will, however, cooperate in criminal ventures for profit.

In addition to drug trafficking they are involved in auto theft, gun violations, assaults and larceny from automobiles.

Latin Kings:
A predominantly Hispanic street and prison gang with two major factions: one in Chicago and one in New England. These gangs started as social groups in Hispanic communities but later evolved into organized criminal enterprises involved in drug trafficking and violent crime. Latin Kings is a very structured gang that relies on strict, detailed charters to maintain discipline. The Chicago-based Latin Kings faction is the foundation upon which all Latin Kings gangs are based. The gang operates drug distribution enterprises on the North and Southeast Sides of Chicago and has expanded throughout Illinois and into other states, including North Carolina. The New England-based Latin Kings faction started in the Connecticut prison system in the late 1980s as an offshoot of the Chicago-based Latin Kings. Latin Kings has attempted to consolidate the Chicago- and New England-based factions.

Others:

Gangster Disciples
 UBN,  the Black Gangster Disciples,
Pegans, Skinheads, Mexican Mafia,
Ñetas,  The Mexican Boys, Los Vagos (The Lazy Ones),
Los Traviesos (The Troublemakers), Los Pitufos (The Smurfs)
 
Asian Gangs:
 Fuk Ching gang in Chinatown NY
 Southeast Asian Gangs
 Asian Dragon Family
 SnakeHeads
 Dragon Family 
 Vietnamese gangs were on the rise in the 1980's
 Cities with large Vietnamese populations such as Philadelphia, New York City,
Chicago, Houston, New Orleans, Boston, Los Angeles and Washington, D.C.,
all had gangs.

Colombo - Narcotics, gambling, loan-sharking, cigarette smuggling, pornography, counterfeiting, hijacking, bankruptcy fraud, and so on.
Carmine 'The Snake' Persico

Gambino - Narcotics (including heroin operations ranging from Sicily to Asia), gambling, and car theft.
Carlo Gambino, Paul Castellano and John Gotti

Genovese - Arguably the most powerful and wealthy crime family in the New York area, and perhaps the entire country, the Genovese family maintains major muscle in narcotics, loan-sharking, extortion rackets, pornography, labor union racketeering, restaurants, seafood distribution and vending machines.
Charles "Lucky" Luciano, Vincent "the Chin" Gigante, Dominick "Quiet Dom" Cirillo

Lucchese - Narcotics, gambling, loan-sharking, waste management, construction, and involvement in the garment industry.
Anthony "Tony Ducks" Corallo

Timeline Arrests, Famous Hits (rub outs, whacks, murders):

In the 1800's, New Orleans was the largest Mafia site in the United States. It was while investigating the murder of an Italian immigrant that the current Police Chief, David Hennessey discovered the existence of this secret society. Chief Hennessey was assassinated before this murder case could go to trial.

1929 Valentine's Day massacre. Alphonse "Scarface" Capone gansters machine gunned seven of Bugs Moran's men in a Chicago warehouse.

1930's - Originally a society of mostly Jewish hitmen, Murder Inc. eventually became the American Crime Syndicate's rub-out wing. In the early 30's, as Lucky Luciano was building his empire, his associates Meyer Lansky and Bugsy Siegel started Murder Inc. as a way to deal with "problems." In Brownsville, Brooklyn, at the corner of Livonia and Saratoga Avenues, Midnight Rose's candy store housed some of the most lethal for-hire contract killers, consisting mainly of Jews and Italians, that this country has ever known. These men, dubiously named Murder Incorporated by the press, carried out over eight hundred contract murders while sharing egg creams and betting on Dodger games at Ebbets Field.

1935 - Dutch Schultz and several of his men were killed in the Palace Chop House and Tavern in Newark New Jersey while discussing a plan to Kill New York District Attorney, Thomas E. Dewey; The crime syndicate did not agree with his plan.

1947 - Benjamin "Bugsy" Siegel was killed by Charles "Lucky" Luciano's hit men because he refused to repay money he borrowed to build the first super casino/hotel in Las Vegas, the Flamingo which was unprofitable.

1972 - Joe "Crazy Joey" Gallo, a local mafia boss, was shot dead as he celebrated his 43rd birthday at Umberto's Clam House at the northern edge of Little Italy on Mulberry Street.

1985 - Paul Castellano, boss of the Gambino family, was gunned down by John Gotti's men while standing outside the Sparks Steak House restaurant on East 46th Street.

1992 - John Gotti is convicted on RICO (1970 Racketeer-Influenced and Corrupt Organization Act) charges. "Sammy the Bull" Gravano's testimony was intrramental.

1997, when Salvatore "Sammy the Bull" Gravano came out with a book in which he admitted killing Stillitano and 18 other mobsters, including a Trenton capo.

The National Mafia Commission banned mob hits in the early 1990's. So many mobsters were facing life long jail terms, and; the feds were getting so much leverage against the mob with the murder raps, that too many guys were turning state's evidence. The mob called a moratorium on hits.

See:
Italian Organized Crime at the Nathanson Centre of NYU
New York Five Families
Famous Chicago Mob Hits
History of the Mafia - 1950-1986

Crime Fighting:
New York law enforcement officials were nationwide pioneers in using the Racketeer Influenced Corrupt Organizations (RICO) statute to go after street gangs, which they have done with a great deal of success. It is more difficult to prossicute omplicated RICO laws to get harsher penalties. Source: Senator Schumer press report.

Los Angeles Gang Problems:
A 2005 NBCActionNews report states:
There is an estimated membership of 100,000 youths and young adults in Los Angeles County alone.

More than one-third of the nation's gang-related killings occur in Los Angeles County, according to a Scripps Howard News Service study of homicides reported to the FBI from 1980 through 2002. Seventy-one percent occurred in California. Both trends have held steady throughout the 23-year period.

A 2002 report on gangs in Los Angeles states:
"The LAPD says Hispanic gangs far outnumber black gangs, with a membership of more than 34,000 versus nearly 19,000.
The murder tally in Boyle Heights over the last three months is around 20."

See:
Drugs and Gangs fast facts at the National Drug Intelligence Center of the DOJ.
Nathanson Centre for the study of Organized Crime and Corruption at NYU.
Murder rates in Large US Cities.
The American "Mafia" - Who Was Who