These are very high quality with automatic suction-and-release cycles. A hospital-grade breast pump is the best pump to use if you are trying to establish and maintain a milk supply for a baby that isn't able to breastfeed. They are heavy and expensive (you can buy or rent), but they are very effective. No breast pump can effectively empty your breasts as good as baby, but these come closer than any other pump.
Portable double-electric pumps
These are much less expensive than hospital-grade pumps, but are a little less effective. They work well for many working moms who are pumping daily to provide breast milk for the times that they are away from baby. Portable double-electric pumps are lightweight and designed for convenience. They often come with tote bags, accessories, battery packs, and car-lighter adaptors. Plus, you can pump both breasts simultaneously.
Lower-cost electric pumps
These pumps can be plugged in or used with a battery pack, and some varieties allow for double pumping. Lower-cost pumps are usually lightweight, but noisy. They work fine for moms who only pump occasionally but not a great choice for moms pumping daily. They have smaller motors that can wear out easily and aren't as effective at maintaining your milk supply as varieties with stronger motors.
Hand pumps (aka manual pumps) have no motor — milk is suctioned out by mechanical means. The most common hand pumps involve two plastic cylinders that work to create suction in a way similar to a syringe. Others use other mechanical means, and some can be used with one hand (if you have two, you can double pump). These are okay for occasional pumping but aren’t for moms who pump more than once per day.
"Bicycle horn" manual pumps can damage your breasts and should not be used.