Gateways
Any device that translates one data format to another is called a gateway. Some examples of gateways include a router that translates data from one network protocol to another, a bridge that converts between two networking systems, and a software application that converts between two dissimilar formats. The key point about a gateway is that only the data format is translated, not the data itself. In many cases, the gateway functionality is incorporated into another device.
Gateways and Default Gateways
Don't confuse a gateway with the term default gateway, the term default gateway refers to a router to which all network transmissions not destined for the local network are sent.
CSU/DSU
A Channel Service Unit/Digital Service Unit (CSU/DSU), sometimes called Data Service Unit, is a device that converts the digital signal format used on LANs into one used on WANs. Such translation is necessary because the networking technologies used on WANs are different from those used on LANs.
The CSU/DSU sits between the LAN and the access point provided by the telecommunications company. Many router manufacturers are now incorporating CSU/DSU functionality into their products.