IIS is the Web Server software from Microsoft. Normally IIS is a part of Windows OS system, it means you do not need to purchase an IIS separately.
The IIS version and Windows system version matches (Due to the record from Wikipedia):
IIS 1.0 was a free add-on for Windows NT 3.51.
IIS 2.0 was included with Windows NT 4.0.
IIS 3.0 was included with Windows NT 4 SP3
IIS 4.0 was released as "Option Pack" for Windows NT 4.0.
IIS 5.0 shipped with Windows 2000
IIS 5.1 was shipped with Windows XP Pro
IIS 6.0 was included with Windows Server 2003 and Windows XP Pro x64 Edition
IIS 7.0 was a complete redesign and rewrite of IIS, and was shipped with Windows Vista and Windows Server 2008.
IIS 7.5 included in Windows 7 and Windows Server 2008 R2.
IIS 8.0 is only available in Windows Server 2012 and Windows 8.
And, we must know the IIS version is for corresponding Windows OS version, for example: IIS 7 can not installed on Windows server 2003; and, IIS 6 can not installed and run on Widows server 2008 and Windows 7;