The 2 main groups are electric pumps and manual pumps. The only decent electric pump I ever used was provided by the hospital when I had my own C-section. The ones I bought for home use werent worth a flip.
The manual pump that I liked the best had one cylinder inside another with a rubber seal to make the suction, and a funnel to place over the breast. It takes a little time, and it is advisable to re-adjust your breast placement on occasion as your breast will shift in it as air bubbles are released. And dont forget to massage your whole breast a few times during the process.
Also, make sure that you can also hand-express. If you do this BEFORE using the pump, you will empty the milk that is in the nipple, and immediately behind the aeorelia (sp?) and then the pump wont be as painful when you are really full.