To supplement what allworxguy already said…
In the very first releases of the software, the Transfer feature (F70) didn't exist. Calls were transferred using the Conference/Transfer feature (F3).
Basically, you would call the extension where you wanted to transfer the call (which puts the caller on hold). When the person at the target extension answered, you could announce the call. You would then press the Conf/Trans button and the held line button (which creates a conference). You would finally press the Rls button (which removes yourself from the conference, leaving the caller and the target extension connected).
This kind of transfer is referred to as Transfer using Conference. Even with the introduction of the Transfer feature, calls can still be transferred using conference.
There is another way to transfer calls that is referred to as Transfer using Hold. This kind of transfer happens when a call is put on hold at one extension and is picked up by pressing the flashing line button at another extension.
Some documentation, such as the Norstar Handbook, even count up to 4 different types of transfer:
[ol 1]
[li]Transfer Immediate (F70)[/li]
[li]Transfer with Announce (F70)[/li]
[li]Transfer using Conference (F3)[/li]
[li]Transfer using Hold[/li]
[/ol]