Let me first say, Medela is the pump preferred by most nursing moms, and the easiest to purchase/rent and buy accessories for, so many of the examples given will be of Medela pumps. There are many other brands, since now every company that makes kids' stuff has created a pump, but most of them don't stand up to Medela. They are cheaply made, hence the much cheaper prices on most of them, and don't compare in the quality and ability of the Medela or even Ameda pumps.

There's 4 kinds of breastpumps:

1. single manual
2. single electric
3. double electric
4. double electric hospital grade

1. Single Manual (hand powered) Pump

If you do not plan to pump often (less than once daily) then this may just be the way to go. I also carried a single hand pump in my diaper bag so I could pump as frequently as I needed to to keep supply up.  These are the most inexpensive of the types of pumps, and Medela's Harmony pump is excellent. It actually has 2 phases of pumping, one to simulate the let-down phase, and another for simulating the expression phase. Both completely controlled by your hand.

2. Single Electric

This would be a step up in efficiency from the manual single pump, but still not good for frequent pumping, since it's a single pump and would take double the time (usually at least a half hour) to do each breast separately.  If you're going to spend the money on an electric pump, I would say spend an extra $30 or so and make it a double electric of the same kind. Medela's single select electric is $77, the double is $120. Not a huge difference in comparison to the time saved. A single electric pump may be just what you need if you baby refuses to nurse on one side.

3. Double Electric

Double electric is what you need for frequent (several times daily) pumping, like if you are going back to work or plan to exclusively pump and bottlefeed pumped milk.  A double electric pump allows you to do both breasts at the same time, and offer control of suction and speed on many models.  Leader of the pack here is Medela's Pump In Style. But there are others: Avent Isis, Avent Iq, Evenflo Elan Double. The only one comparable to Medela's Pump In Style is Ameda's Purely Yoursm which at only 1lb. is great for convenience.

4. Double Electric Hospital Grade

I personally think any mom new to breastfeeding should have a hospital grade pump rented and ready for them when they get home from the hospital with baby. It will deter any supply issues, as hospital grade is the only one made especially for building milk supply.  Rental ranges from $30-$70 (at least in my area!) and once a supply is established about a month and a half after birth, you can send it back and use a personal use pump, whether single, double, electric, or manual.