If one site runs more than one server they must use different port numbers. Alternatively, several hostnames may be mapped to the same computer in which case they are known as "virtual servers".
Apache, IIS, and nginx are three popular web servers. There are many others including some for practically every platform. Servers differ mostly in the "server-side" features they offer such as server-side includes, and in their authentication and access control mechanisms. All decent servers support CGI and most have some binary API as well.