What is a Whip?

A "whip" is an appointed member of a party whose job it is to try to make party members cohesive.

From http://www.parliament.uk/works/pagovopp.cfm#whips:

The duties of Whips include:

• keeping MPs and peers informed of forthcoming parliamentary business
• maintaining the party's voting strength by ensuring members attend important debates and support their party in parliamentary divisions
• passing on to the party leadership the opinions of backbench members.