As of March 2011 there were more than 2 Billion web users (30% of the population) worldwide.

Market Share | Compatibility | Performance | Standards | Add-ons | New Features |
Obsolete tags/attributes

Browser Market Share April 2011

Browser	1995	2000	2005	November 2010	April 2011	Oct 2011	Change /yr *
Microsoft Internet Explorer (IE) (all)	2.9%	79.1%	90.9%	48.2%	44.5%	40%	-17%
9.0 (launched 3/15/11)					2.3%
8.0				29.5%	30.2%
7.0				11.9%	7.8%
6.0 (launched 8/2001)				6.4%	4.1%
Mozilla† Firefox (all)			6.0%	31.2%	29.7%	26%	-15%
4.0 (launched 3/22/11)					7.6%
3.5+				28.5%	20.9%
3.1-				2.3%	1.2%
Google Chrome				13.3%	18.3%	25%	88%
Apple Safari			1.0%	4.7%	5.0%	5.9%	26%
Opera		0.1%	0.5%	2.0%	1.9%	1.8%	-10%
Others				1.0%	2.9%
Netscape †	80%	19%	0.1%
AOL‡		‡	‡
SeaMonkey †				0.5%

The first Microsoft browser was licensed from Spyglass who delivered two versions of Mosaic to Microsoft. It was modified and released as Internet Explorer (IE) in 1995.

† - Mozilla was the internal codename for the development of the Netscape Navigator browser in 1994. The name is formed from "Mosaic killer"; Mosaic was the public domain browser developed by the National Center for Supercomputing Applications (NCSA) at the U. Ill. in 1993.
Mosaic was the first browser to display images inline with text instead of displaying images in a separate window and is credited with popularizing the World Wide Web.

Firefox and the Firefox Logo are a trademark of the Mozilla.
SeaMonkey is a registered trademarks of the Mozilla Foundation.

‡ AOL bought Netscape in 1999.
In 2002, according to W3Schkools.com, AOL had a 5% browser market share and netscape had an 8% share. I couldn't find any other data for AOL.
In 2003, AOL signed a seven-year contract with Microsoft to use Internet Explorer layout engine in their products and as such AOL Explorer.
In 2008 AOL dropped support the Netscape browser.

See: Browsers at Internet History for more on the "browser wars" and the antitrust suit against Microsoft.

April 2011 Stats - Other sources

				*
StatCounter	44.5%	29.7%	18.3%	5.0%	1.9%
Net Applications	55.1%	21.6%	11.9%	7.15%	3.3%
W3Counter	37.8%	29.5%	15.7%	6.2%
Wikimedia	38.7%	26.1%	14.7%	5.9%	4.1%
W3schools	24.3%	42.9%	25.6%	4.1%	2.6%

Mobile (phone/tablet) browsers accounted for 5% to 7% of usage in April 2011 .

Tablets: In June 2011 tablets accounted for a 1% market share.

iPad                0.92%
Samsung Galaxy Tab  0.018%
Motorola Xoom       0.012%
Blackberry PlayBook 0.003%

Count by OS: at NetApplications (NetMarketShare.com) May 2011
Windows 88.7%
Mac      5.3%
iOS      2.4%
Java ME  1.2% (Java Micro Edition for mobile devices, set-top boxes, ...)
Linus    0.9%
Android  0.8%

Performance:
There are many ways to measure browser performance, CSS rendering speed, javascript speed, page load time, CPU usage, application load time, ...

In May 2011 Daniel Bailey at Mozilla: Browser Benchmarks Suck! | ConceivablyTech says,
"Taras Glek, a developer at Mozilla, recognizes that Mozilla has "traditionally relied on benchmarks (such as Talos, Sunspider or Kraken) to make Firefox faster, but now says that benchmarks "suck".

Source: What Web Browser is Fastest for Windows in 2011? - Benchmarking JavaScript, Flash and HTML 5 Performance on Internet Browsers - Legit Reviews

Others:
Peacekeeper - The Browser Benchmark from Futuremark Corporation

Web Browser Performance 2009 at SixRevisions.com

Web Standards Support:
When Netscape 4 and IE 4 implemented Cascading Style Sheets (CSS), around 1997, their support did not match the W3C standard (or, indeed, each other). Starting with IE 6 (8/2001) Microsoft started to implement the standards.
Many web pages had to have exceptions (quirks mode) for IE 5 and below.
With IE 5 at < 0.1% of market share now, these exceptions are being dropped.

HTML5 is an evolving standard, but as April 2011 IE 9, Firefox 4, Chrome 11 and Safari 5.1 had implemented 85% of the proposal.
They keep leapfrogging each other. Safari was ahead until Firefox 4. Safari is expected to retake the lead with 5.1, then Firefox with version 5. Chrome 12 will probably put it ahead. See:
HTML5 & CSS3 Support, Web Design Tools & Support - FindMeByIP
The HTML5 test - How well does your browser support HTML5?
CSS3 & HTML5, Digital Marketing, Analytics & Deep Blue Sky
The HTML5 test - How well does your browser support HTML5?

Browser or Layout engines:
Most web browsers are modular with a user interface component and layout engine. The layout engine (sometimes called a browser engine or rendering engine) takes the web page and processes the content, HTML, XML, Images, CSS, XSL, etc, to format it for the screen. They are shared by different browsers, e-mail clients and on-line help systems. Common ones are:
Gecko: Firefox, SeaMonkey
Trident: Internet Explorer, Outlook, Real Player
KHTML by Konqueror - KDE: Safari, Chrome
Presto: Opera
See: Web browser engine - Wikipedia,
and Technology News: Developers: KHTML vs. Gecko vs. Trident vs. Presto: Behind the Browser

New Features:
IE 9 and Firefox 4, both introduced in March 2011, let you block sites from tracking your movements online (via a tracking cookie, for example). These are primarily used by third parties (another web site which provides adds to the site you are visiting) and are used to target adds to things you seem to be interested in.
However, the blocking has some problems, Firefox relies on the tracking Web sites to play nice and Microsoft IE9 requires user's to identify and block Web sites on a case by case basis.
The United States Federal Trade Commission (FTC) is looking for better solutions.
See: Privacy and third-party cookies at Wikipedia

Most major browsers, including IE9 and Firefox 4, now feature simplified interfaces with fewer toolbars than before, and take up less of the screen than older browsers.

See: Compared: IE9 and Firefox 4 Release Candidate | PCWorld

Add-Ons, Plug-Ins and Extensions:
Plug-Ins are applications like Adobe Acrobat pdf readers which will open in a browser window.
Add-Ons and Extensions are small bits of code which add functionality or change the look and feel of the browser itself.
They are most popular in Firefox and Chrome. SeaMonkey also accepts most add-ons for Firefox.
The most popular are Add blockers which eliminate most adds from the web pages you access. See:
Web Browser Add-Ons, Plug-Ins, Extensions, Skins, and More at about.com
Firefox add-ons here.
Dave McRitchie's extensive Firefox Extension List
Chrome Extensions at chrome.google.com/webstore and ChromeExtensions.org

Deprecated (Obsolete) Tags/Attributes:
Many html tags (e.g. Codes in web pages for formatting text, e.g. underline, center, frames, font size and face, ...) were eliminated in HTML 4 which was the current standard in 2011. However, most browsers continue to support them and will probably do so according to current until the end of the decade (2018-2020). They are being replaced by other HTML codes and Cascading Style Sheet (CSS) controls. See:
Deprecated HTML tags and alternatives at CodeHelp.co.uk

Links:
Peacekeeper - The Browser Benchmark from Futuremark Corporation
Usage share of web browsers - Wikipedia
Comparison of web browsers - Wikipedia
Compare Security Features in Internet Explorer with and without Secure IE Browser Security
Browser Statistics at w3Schools.com
Browser usage stats - March 2011: Tech News
CNET Browser Reviews
Timeline of Web Browsers
Internet History