  Nilsequences arose in the study of the multiple ergodic averages associated
to Furstenberg's proof of Szemer\'edi's Theorem and have since played a role in
problems in additive combinatorics. Nilsequences are a generalization of almost
periodic sequences and we study which portions of the classical theory for
almost periodic sequences can be generalized for two step nilsequences. We
state and prove basic properties for 2-step nilsequences and give a
classification scheme for them.
