iis web server is the server on most windows platforms, it supports asp, dot net, mssql server, can also support (if you install) perl and php

apache web server is the server fo linux platforms, it supports, perl, php, mysql server and python

most ISP that offer hosting will use both servers, because they want to offer both platforms based on the clients needs

if you are just starting out to host a web site on the net and u are trying to decide what platform to use, most people go with apache because it is bullet proof, but it also means you have to learn linux (sure there are windows versions of apache but the are prone to windows problems)

if you want to make your life easy, you can go with IIS on you windows machine but you have to make sure you have all the security patchs installed, microsoft has relesed a software called MS Base line security checker, that can test the iis server and tell you how to tighten security, be very carfull, if you want a true iis web server for the inet, make sure you have a legit ms server operating system, make sure you are behind a firewall and that you have the right ports open and redirected to the server machine... be very carfull make sur you know what you are doing, you do not want to put a server machine on the net that has a lot of securty holes in it, hackers will break in and wreck havoc with you

appache is much more secure, (that is apache in the linux enviroment) but you still need to know what u are doing if you want to put a web server on the net