Basically, it's like x-mas at grandmas house. :O)
You join in a swap (say...the x-mas in july swap going on now). Each swap has a different theme, and a different "host". Each swap has a different time limit. It will be posted at the beginning of each swap "swap is open for up to 20 people, we will take sign ups until june 15th..etc".

At that time, the "host" will assign partners. Then you will send your items to the partner assigned to you. You will send the items specified in the swap (seeds, plants, homemade goodies..etc...it's totally up to you)

That's basically all there is to it.

And it's alot of fun. :O)
I hope that helps you a little. I'm pretty new to this too!