Apache or Apache HTTP web server
Apache or Apache HTTP web server, is an open source Web server application managed by the Apache Software Foundation. The server software is freely distributed, and the open source license means users can edit the underlying code to tweak performance and contribute to the future development of the program which is the most popular HTTP server in use today.

Internet Information Services (IIS)
IIS is Microsoft’s web server offering, playing second fiddle to market leader Apache. IIS is second only to HTTP as the most used HTTP server in the world.
As is expected of a core Microsoft product, it only runs and is bundled on Windows operating systems, but is otherwise free for use. It is a closed software product and supported by solely by Microsoft.
The latest version of IIS, version 7.0, is included in Windows Server 2008. It builds on the foundation of IIS 6.0 and introduces a number of new features. One of the most important changes is that IIS 7.0 has a fully modular architecture. This feature lets users install or uninstall discrete pieces of functionality and also gives users the ability to leverage new modules from Microsoft and from the Web development community at large.

If you host a website, chances are good that you are running either Apache or Internet Information Services (IIS). They are by far the two most common web server platforms, between them commanding about 70% of the market.

It consists of a series of services including File Transfer Protocol (FTP), Hypertext Transfer Protocol (HTTP), Simple Mail Transfer Protocol (SMTP) and others that enable a Windows machine to manage Web sites.
The latest versions also includes various modules for security,
Summary:
1. Apache is free while IIS is packaged with Windows.
2. IIS only runs on Windows while Apache can run on almost any OS including UNIX, Apple’s OS X, and on most Linux Distributions.
3. ASPX runs only in IIS.
4. IIS has a dedicated staff to answer most problems while support for Apache comes from the community itself.
5. IIS is optimized for Windows because they are from the same company.
6. The Windows OS is prone to security risks