Many times it can be confusing to know which one it is but you have to neuter a male one and spade a female. But if you are going to go get them spade you really don't have to worry about it because they know what it is and they decide what to do.
For a male dog the procedure it called a neuter, for a female it's called a spay. A neuter is the removal of the testicles, and during a spay the ovaries and uterus are removed.