A WAP gateway is a piece of software that has several functions in the "chain" between the WAP device and the web server. These functions are in general:

• Converting the markup language (WML) from textual format to tokenized (binary/compressed) format which is readable by the WAP device.
• Translating the requests from the WAP device to HTTP requests for the "web" world.
• Convert between the SSL encryption used in the "web" world and the WTLS encryption used in the WAP world.
• Convert between the "transport" protocol of the "web", TCP, and that of the WAP world, WDP.

Optionally many gateways also perform other conversions such as between plain text files, simple HTML files etc into a WAP readable format. If a WAP gateway is hosted by a mobile operator, the mobile operator may use the gateway to add mobile specific information into the HTTP stream such as the subscriber number, location details etc (although this is not widely used).

Some WAP gateways are publicly available on the internet, while others are hosted by mobile operators, and these are usually restricted to accept WAP devices coming from the mobile operator's dial in service only.

A small number of gateways, or gateway-like devices ("WAP Servers") are hosted by the content providers, i.e. the people who own the web servers with the WAP content.

Tantau's website
Chapter 1 in "Professional WAP" from Wrox
Chapter 3 in "Professional WAP" from Wrox