I am about to purchase Sony Acid Pro 5.0 and I still am unclear about the software. Can I write songs and have the computer play the songs I write with a variety of instruments? Or is it just loops that I work with? Please answer. Thanks.

It is loops you use to mix and make a song, score etc. You can use a variety of instruments, but they will all be loops. You can't "write" music.

Oh no....... Are you sure? Can I play music on a MIDI Keyboard and put that onto the program and use that to make music?

I've renamed and moved your thread to the audio forum.

Acid is a loop and sample based system. You can use enyire loops, or create your own loops out of specific sample sounds, If you want a drum track, for example, you can find a loop that works for you, or you can start with individual kit sounds ( kick, snare hats. etc.) and create your own loop.
You can also slice and dice the complete loops and end up with something new and customized. For midi recording, on the other hand, I use Steinberg Nuendo and Cubase SX. If you're looking for something cheaper, check out Cakewalk.
Bruce Yarock

But it says that you can import MIDI files, can you?

Could I buy Cakewalk, do MIDI stuff with that and then put those files into Acid Pro?

As far as importing midi files into Acid 5.0, I can't answer that. I use 4.0 and don't have 5.0.
I do import acid loops,etc. into Nuendo and Cubase, and I'm sure you can do the same in Cakewalk. I like working in these programs more than in Acid... I find Acid's commands etc. to be very non-intuitive,but that's just me.
Bruce yarock