In Tae Kwon Do terminology, a poomse is a form: that is, a predefined sequence of moves, simulating a fight against one or more imaginary opponents.
The name "poomse" is one of two terms used for forms, the other one being hyung. "Poomse" is more commonly used by the World Tae Kwon Do Federation, while "hyung" is more common in the International Tae Kwon Do Federation. The two have different sets of forms which are commonly practiced by students; see the node Taekwondo Hyung for more information.
As a final note, the WTF forms are more commonly sanctioned and used in tournaments. I do not know exactly why this is.