Neither Microsoft Exchange Server nor Web servers are Operating Systems.
Microsoft Exchange Server is a corporate email solution written by the Microsoft Corporation. It competes with Linux / Unix SMTP/POP3 servers and Lotus Notes.
Web servers are what allow people to display web pages which are stored on a server. There are several different web servers available. Microsoft’s web server is called Internet Information Server (IIS). The other major web server on the Internet is called Apache. While IIS can only be installed on computers running the Microsoft Windows operating systems, Apache can be installed on just about any operating system including Mac, Microsoft Windows, Linux, Unix, etc.
The operating systems which are commonly used on servers are Microsoft Windows Server 2003, Linux, and the various flavors of Unix (HPUX, Solaris, etc). For internal servers the Microsoft Windows Server 2000 and 2003 operating systems hold the bulk of the market share. For Internet servers such as web servers and mail servers Unix holds the most market share.
The operating systems which are commonly used on corporate workstations and home computers are Microsoft Windows XP, Microsoft Windows Vista, Mac, and Linux.