WML stands for Wireless Markup Language, and is what HTML is to a web browser. It is based on XML, or rather it is an XML application. If you're going from HTML to WML, with no prior knowledge of XML, chances are you'll find WML *very* strict. Just like HTML and XML, WML is read and interpreted by a browser built into the WAP device. For WAP devices, the browser is commonly called a micro browser, indicating that its capabilities are somewhat limited These capabilities are of course also limited to the capabilities of the WAP device in which it lives.

A good starting point is the OpenWave documentation WML 1.3 Language Reference and WML 1.3 Developer's Guide or the O'Reilly book Learning WML & WMLScript.