I'm looking into setting up a farm. I am wanting 2 web servers, 2 app servers and 1 DB server. I am just wondering if the web servers are just vanilla IIS or is some sharepoint bits in there too?
SharePoint installs on top of ASP.NET, itself built on top of IIS.
Also, some specific modules (to handle Windows authentication, ...) needs to be activated in IIS. All these prerequisites are automatically installed (if needed) by the SharePoint Prerequisites installer.
But SharePoint does not actually modifiy IIS binaries.