WML overview

WML is based on the eXtensible Markup Language (XML) and was developed by the Wireless Application Protocol (WAP) Forum.

WML, like HTML, is a type of markup language used for formatting content in mobile devices. WML is very similar to HTML except for some key differences:

• Cards. With WML, each page, called a deck, is broken down into several small cards. While the deck is downloaded from the server all at once, cards are displayed one at a time. This is done to acquire small screen size. The card specifies a single interaction between the user agent and the user. Cards are grouped together into decks, the top-most element of a wml document. 
• Soft keys. Soft keys are device buttons that can be assigned various functions.
• Wireless bitmap images. Because of the small display area on mobile devices, images tend to be small. Mobile phone displays vary in size from roughly
  80 x 60 pixel areas to 100 x 100, or more.

Since WML is based on XML, it is a very strict language. All tags must be in lowercase and they must all be closed in order for the code to be well-formed and run correctly.
The Wireless Markup Language is a page description language that describes how WAP content is presented to the user. With WML you can display information on a mobile phone, give the user input options, and specify how the user agent should respond when the user activates a user interface function or presses a key.

WML reference