A WAP server is basically the same as a web server hosting web pages. However, instead of hosting .html, a WAP server hosts WML or XHTML MP documents. Some companies providing mobile services use the term WAP server to also denote a WAP Server and a WAP Gateway. Most of the time, a WAP server will only mean a Apache or IIS.
What is WML?
WML is short for Wireless Markup Language. This is the markup language used to build WAP sites. WML is very similar to HTML and those familiar with the latter will have no problems learning WML. WML supports client-side scripting called WMLScript. XHTML MP is actually the next version of WML used in WAP 2. However, certain functions of WML are not found in the new specification. This is why WML is still used today.