A web server is a software that uses HTTP for serving content and files for users in response to their request that are sent by HTTP clients. The content can be provided to LAN clients (for example a corporate Intranet) or WAN clients (web server to host web sites on the Internet).
The most common web servers are:
- Apache - developed by Apache Software Foundation, it supports almost all operating systems (Linux, Windows, Unix, MacOS..). This is an open source software.
- IIS - developed by Microsoft, supports all the platforms (almost all) that run a Windows operating system. IIS is not open source. More info on IIS Internet Information Services.