WML stands for Wireless Markup Language, and it is to a mobile browser what HTML is to a standard web browser. It is based on XML, which means that 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.