Question about trainers

So I thought, since I'm doing this triathlon, I might try to track down a cheap, used bike trainer from Craigslist and use that to practice with clipless pedals. My question is, the trainer is just the part that the back wheel goes into, correct? That black plastic thing in the front that the front wheel sits in is separate? Do you need that thing? I'm assuming so, to keep the bike level. I've only ever seen a trainer at the LBS, which they put my bike on and had me pedal and do various things while they were fitting me to the bike. I just want to make sure I know what I'm looking for. Thanks!

I use a hunk of wood for my front wheel to sit on.

V.

The front riser block is usually separate. I bought one but it didn't end up being the right height to level me out (raised the front too much). Now, I use various wood blocks.

I use telephone books.

Like Michele, when we used a trainer, we used a couple of telephone books too.
We did start with a chunk of wood, but the bike wheel tended to move off it too easily. For some reason, we didn't have the same problem with the telephone books...

(We have a spin bike now)