From Sheldon Brown:
Compact Crank
A double-chainring crankset with that permits the use of smaller chainrings than will fit with the common 130 mm B.C.D. Most "compact double" cranks use the old standard 110 mm B.C.D., which permits the use of chainrings as small as 33 teeth (more commonly, 34 or 36 teeth.)
Compact crank sets usually come with a 50 tooth chainring, and are normally used with an 11 or 12 tooth top sprocket in back.
110 mm B.C.D. double cranks with full-sized chainrings (52-42, 52-40, etc.) were common in the late'70s and early '80s, but they had become nearly extinct for double chainrings. The rebirth of this format, with smaller rings, was pioneered by Tyler Hamilton who used one of these in the 2003 Tour de France
http://www.sheldonbrown.com/gloss_cn...l#compactcrank
__________________
The Lord is merciful and gracious, slow to anger and abounding in steadfast love. - Psalm 103:8
I am a cyclist. I am not the fastest or the fittest. But I will get to where I'm going with a smile on my face.