The Open Guide to London: the free London guide

Big Ben is the common term for the Westminster Clock Tower, part of the Houses of Parliament. Big Ben was originally the name of one of the bells, and was supposedly named either after Sir Benjamin Hall, the Commissioner of Works at the time (and a large man), or Ben Caunt, a popular boxer in 1858, when the bell was cast.

Although not generally open to the public, you can arrange a tour through your MP: http://www.parliament.uk/directories/hcio/clock_tower.cfm