Wireless Markup Language (WML) is an XML document prepared to format a webpage in a mobile browser. It was designed in the standard WAP 1.0 and was deprecated by XHTML MP. Many new devices/phones supports this language right now too.
The information is distributed in decks (a WML file) composed by cards (a visual webpage). Inside one deck, you can include many cards and you can link each other reducing server traffic.
It also supports images (in WBMP format), tables, forms and an script language, called WMLScript. It doesn't support color screens nor many advanced design features. XHTML MP (Mobile Profile) solves this problem using CSS.
You can test WML pages with many Nokia emulators and SDKs.
This is a sample of WML file:
<!DOCTYPE wml PUBLIC "-//PHONE.COM//DTD WML 1.1//EN"
<card id="main" title="First Card">
<p mode="wrap">This is our first mobile webpage
<card id="card2" title="Second Card">
<p mode="wrap">This is our second card</p>