The industry term for these devices is a Network Interface Card (NIC).

Unless you have a device with a seperate power source (such as an AC adapter), you should consult the FAQ section on Network Interface Cards.

If you have a device with a wall-powered source (e.g. an AC adapter that plugs into an electrical socket), your device probably has more in common with a Router. Consult the section on Routers/Gateway devices.

If your device is also wireless, you should also see the section on Wireless Routers.