IIS stands for Internet Information Services (previously named Internet Information Server) an extensible web server made by Microsoft for use with the Windows NT series, and has been an integral part of that family of products since NT version 4.0, and was initially offered as a free add-on Windows NT version 3.51.
As of IIS 7.0, IIS features a modular architecture, in which extensions can be removed or added individually, so that only modules for specifically desired functionality need to be installed. By default, as of version 7.0, a full install of IIS comes equipped with modules enabled for security, caching, compression, content and logging and diagnostic modules. IIS 7.5 added enhanced security features like IP security, request filtering, client certificate mapping and URL authorization, while IIS 8.0 offered new features designed to improve performance and ease administration, including support for centralized SSL certificates, the WebSocket protocol and CPU throttling that enables the server administrator to control the CPU usage by each application pool in order to optimize performance.
IIS is reportedly the second most popular web server in use today, coming in behind only Apache HTTP Server, with a market share of 19.45% as of July 2013