Electric breast pumps are offered as a single-breast model or a double-breast model. With a single-breast model you can only pump one breast at a time. A double-breast pump allows you to pump both breasts at the same time and you can also pump one breast at a time if you need to.
Manual breast pumps are exactly as it sounds, you manually have to pump your breasts by squeezing a hand pump. A manual breast pump is great for on the go if you don’t want to bring your electric pump when you go out and you know you will need to pump. They are much more discrete than the electric breast pumps. I always keep my manual breast pump in the diaper bag just in case. It is small and easy to use if my electric breast pump is unavailable.