From their website:

What you do!
Open a FREE account.
Choose from over 50 products.
Upload your artwork, music or content and design your products.
Set your retail price over the CafePress.com base price.
Promote your CafePress.com shop.
What Cafe Press does!
Manufactures your products as they are ordered.
Ships your products worldwide.
Manages all payment processing (credit cards, checks.)
Provides all customer service (online and phone).
They offer a free two week Premium trial, but I'd go for the free version unless you want to offer books or CDs- you can always upgrade later.
I have two shops there - JaZilla, for digital and fine art, and Speech Zone, for rude sociopolitical commentary.
If you sign up to sell stuff and make a store, my referral word is jazilla. It won't take a cut from your profits but it will help me a lot, and I'd really appreciate it!