There are many different network protocols in use today. For example the large internet company called AOL has its own special email protocol.

If your computer does not use the AOL protocol (and the chances are it won't). Then how do you get email to your friend on the AOL network?

Answer: A gateway.

A gateway converts the data passing between dissimilar networks so that each side can communicate with each other. i.e converts data into the correct network protocol.

The gateway is a mixture of hardware components and software.

This is unlike a standard 'Bridge' which simply joins two networks together that share the same protocol.