The usage share of web browsers 
is the percentage (%) of viewers at a group of websites who use a particular web browser. For example, when it is said that Internet Explorer has 7% usage share (April 2018), it means that some version of Internet Explorer is used by nearly 7 of every 100 visitors who visit a given set of websites.

Accuracy issues 
It can be misleading to measure the browser usage in the number of requests (page hits) made by each user agent.

Overestimation 
Not all requests are generated by a user, as a user agent can make requests at regular time intervals without user input. In this case the user's activity might be overestimated. Some examples:

A web browser that refreshes the webpage at a regular time interval.
A feed reader that requests the RSS or Atom feed at a regular time interval.
Extra files like CSS hacks and JavaScript hacks are often sent to Internet Explorer.
Gecko-based browsers (such as Firefox) can prefetch linked web pages, potentially increasing hits. Link prefetching in Gecko-based browsers is used on pages with enhanced markup, including Google search results.
A user who revisits a site shortly after changing or upgrading browsers may be double-counted under some methodologies; overall numbers at the time of a new version's release may be skewed.
Certain anti-virus products fake their user-agent to appear to be popular browsers. This is done to trick attack sites that might display clean content to the scanner, but not to the browser. The Register reported in June 2008 that traffic from AVG Linkscanner, using an IE6 user-agent, outstripped human link clicks by nearly 10 to 1.
Many types of software, such as Web validators or crawlers, fetch web pages, and send fictitious user-agent strings to appear more like normal traffic.
Browsers such as Safari maintain a dashboard of frequently visited sites. This generates hits to those sites on browser start or at times on launch of a new tab, in order to update the thumbnail images.

Underestimation 
It is also possible to underestimate the usage share by using the number of requests, for example:

A graphical browser typically results in more hits than a text browser, as it downloads files referred to by the HTML document (e.g., images, CSS, and JavaScript).
Many browsers and download managers spoof a different user agent string to the web server to prevent erroneous or malicious browser sniffing which could result in receiving broken or incompatible code, or being completely blocked, and thus increasing the statistics for other browsers (as an example, prior to version 9, the Opera web browser had "Identify as Internet Explorer" as the default user setting). Sometimes you can detect these spoofing browsers based on other things but some people do not do it. (An example is Internet Explorer Mobile which ships in Windows Mobile 6.1.4 and 6.5 with the default user agent string as IE6 Windows XP. Internet Explorer Mobile can be identified by looking at the other HTTP Headers)
Text-based and audio-based browsers do not download any web bugs at all.
Gecko-based browsers since Firefox 1.5 and Opera use fast Document Object Model (DOM) caching. JavaScript is only executed on pageload from net or disk cache, but not if it is loaded from DOM cache. This can have an impact on JavaScript based tracking of browser statistics.
 Some browser extensions and security programs (such as the popular Firefox extension NoScript) block JavaScript, Adobe Flash, Applets and some forms of HTML (HTML blocked through ClearClick) for non-whitelisted sites. This may also affect JavaScript based tracking of browser statistics.
Many browsers accessing a site via one proxy are apt to be counted as only one access if the proxy caches the result; it is usual for browsers on mobile phones in particular to access the web via proxies (many of which violate HTTP's specifications for proxies; even if a web bug is designed to work despite proxying, the proxies run by mobile operators may well mislead it).
While most browsers generate additional page hits by refreshing web pages when user navigates back through page history, some browsers (such as Opera) reuse cached content without resending requests to the server.
Generally, the more faithfully a browser implements HTTP's cache specifications, the more it will be under-reported relative to browsers that implement those specifications poorly.
Plug-ins may change, or append text to, the User Agent string. This can lead to Browser Detection flaws.
User Agents are not guaranteed to be a certain format. As an example of the inconsistency, almost every User Agent pretends to be Mozilla 5.0.
Some ISPs, mainly mobile phone carriers, have begun stripping the user agent strings.

Summary table 

The following table summarizes the usage share of browsers for all platforms.

Historical usage share

Net Applications (2004 Q4 to present) 

Netapplications bases its usage share mainly on the usage statistics of 40,000 websites having around 160 million unique visitors.

On 2009-08-01, Net Applications began weighting its raw data based on the number of internet users in the countries concerned, using data from the CIA ; the changes were applied retroactively to older data. The table below uses weighted data from 2007 Q4 onwards.

W3Counter (May 2007 to present) 
This site counts the last 15,000 page views from each of approximately 30,000 websites.
This limits the influence of sites with more than 15,000 visitors on the usage statistics.

StatCounter (July 2008 to present) 

Statcounter statistics are directly derived from hits (not unique visitors) from 3 million sites using Statcounter totalling more than 15 billion hits per month. No artificial weightings are used.

StatCounter Europe (July 2008 to present)

StatOwl.com (September 2008 to present)

Clicky (Late September 2009 to present)

Older reports

Wikimedia (April 2009 to June 2010) 
Wikimedia squid reports are based on server logs of about 4 billion page requests per month, based on the user agent information that accompanied the requests. These server logs cover requests to the following Wikimedia Foundation projects: Wikipedia, Wikimedia Commons, Wiktionary, Wikibooks, Wikiquote, Wikisource, Wikinews, Wikiversity and others.

TheCounter.com (2000 to 2009) 

TheCounter.com identifies sixteen versions of six browsers (Internet Explorer, Firefox, Safari, Opera, Netscape, and Konqueror). Other browsers are categorised as either ‘Netscape compatible’ (including Google Chrome, which may also be categorized as 'Safari' due to its 'Webkit' subtag) or ‘unknown’. Internet Explorer 8 is identified as Internet Explorer 7. Monthly data includes all hits from 2008-02-01 until the end of the month concerned. More than the exact browser type, this data identifies the underlying web rendering engine used by various browsers, and the table below aggregates them in the same column.

OneStat.com (April 2002 to March 2009)

ADTECH (Europe, 2004 to Q4 2009)

AT Internet Institute (Europe, July 2007 to June 2010) 
AT Internet Institute was formerly known as XiTi.

Methodology: Only counts visits to local sites in 23 European countries and then averages the percentages for those 23 European countries independent of population size.

WebSideStory (USA, February 1999 to June 2006)

GVU WWW user survey (January 1994 to October 1998)

EWS Web Server at UIUC (1996 Q2 to 1998)

ZD Market Intelligence (USA, January 1997 to January 1998)

Zona Research (USA, January 1997 to July 1998)

AdKnowledge (January 1998 to June 1998)

Dataquest (1995 to 1997)

International Data Corporation (USA, 1996 to 1997)

Related pages
List of web browsers
Usage share, Market share

References

Other websites
Chuck Upsdell's Browser Stats lists web stats from many different stats sources and gives the approximate current usage share of the most popular layout engines.
W3Schools' Browser Statistics lists the web stats only from the W3 Schools site, which gives the approximate usage share of browsers among "people with an interest for web technologies."
Wikimedia's most recent squid report lists web stats for Wikipedia and its sister projects.

ru:Браузер#Рыночные доли