Telegraph key is a general term for any switching device used primarily to send Morse code

Morse code

Morse code is a method of transmitting textual information as a series of on-off tones, lights, or clicks that can be directly understood by a skilled listener or observer without special equipment...

. Similar keys are used for all forms of manual telegraphy

Telegraphy

Telegraphy is the long-distance transmission of messages via some form of signalling technology. Telegraphy requires messages to be converted to a code which is known to both sender and receiver...

, such as in electrical telegraph

Electrical telegraph

An electrical telegraph is a telegraph that uses electrical signals, usually conveyed via telecommunication lines or radio. The electromagnetic telegraph is a device for human-to-human transmission of coded text messages....

 and radio telegraphy.
Since its original inception, the telegraph key's design has developed such that there are now multiple types of keys.
A straight key is the common telegraph key as seen in old movies. It is a simple bar with a knob on top and a contact underneath.