From the Wikipedia page at [1]:
'The London congestion charge is a fee charged for some categories of motor vehicle to travel at certain times within the Congestion Charge Zone (CCZ), a traffic area in London. The charge aims to reduce congestion, and raise investment funds for London's transport system. The zone was introduced in central London on 17 February 2003, and extended into parts of west London on 19 February 2007.
The TfL page is [2].