There are lots of types of DJs. There are ones who merely select what to play next, IE your radio DJ or even whoever is controlling what is being played at a party for example.
There are DJs that mix, like in clubs or even at home and record for later use.
Or there is also hip hop DJs which create music for people to rap (or not) over.
All of these do control what you hear usually music recorded beforehand.
The equipment is completely subjective based on what kind of DJing your referring to. For example, at home for your friends, I suppose all you would need is a stereo and a few CDs and you would be considered a DJ. But a DJ doing something professional would need some more equipment, a couple of turn tables, some vinyls, amp, and a mixer; but it is not at all limited there.
Wikipedia can probably tell you all that you would want to know
http://en.wikipedia.org/wiki/DJing