Well it is simple, but like all things related to computers there are some "gotchas".
Firstly, until about 2 years ago most devices in a PC (like hard drives, DVD drives etc) connected to the motherboard by a connector called IDE.
But about 2 or so years ago a new type of connector came out called SATA.
So your existing DVD Drive may be IDE or it may be SATA and you need to find out befoe buying a new one.
Secondly, devices connected to a motherboard can either be Master devices or Slave devices. If you had two devices connected to a single IDE cable you would set one as a master and one as a slave.
This master / slave option is usually set by moving a small jumber on the back of the DVD drive.
So you need to make sure that the drive you put in is set to the same setting (master / slave) as the one you took out.
(I dont think master / slave applies to SATA, I have not done a lot of work on SATA drives).
Apart from that I think it is just a case of removing the screws, sliding the old one out and sliding the new one in.