Assistants to the floor leaders who are also elected by their party conferences. The majority and minority whips (and their assistants) are responsible for mobilizing votes within their parties on major issues. In the absence of a party floor leader, the whip often serves as acting floor leader.
Wikipedia's pages on party leaders in the House and Senate has a bit more on the position in each respective chamber. And there's this from the page on Whip (politics).
A whip is an official in a political party whose primary purpose is to ensure party discipline in a legislature. Whips are a party's "enforcers," who typically offer inducements and threaten punishments for party members to ensure that they vote according to the official party policy. A whip's role is also to ensure that the elected representatives of their party are in attendance when important votes are taken. The usage comes from the hunting term "whipping in," i.e. preventing hounds from wandering away from the pack.
This academic paper contains a good description of the development of the whip system in COngress, as well as an analysis of its effectiveness.
- Party Voting and the Institutionalization of the Whip System.
