Legislative committees are normally restricted in their membership, whereas all members are entitled to participate when a legislature meets as a committee of the whole. This device is extensively used in the United States House of Representatives (although not in the Senate) as a means of expediting legislative business. An appointee of the Speaker chairs the proceedings, formal rules are suspended, and a quorum of only 100 is required instead of the 218 otherwise needed. At Westminster committees of the whole have been used for the discussion of matters of especial importance where it is thought that any member should, in principle, be allowed to participate. Thus for centuries prior to procedural changes in the 1960s all bills authorizing taxation or expenditure were first considered by the House of Commons sitting as a committee of the whole. In 1993 the committee stage of the Maastricht bill was dealt with under this procedure.
http://www.parliament.uk/about/how/laws/stages/committee_whole_house.cfm UK Parliament guide to procedure.