When the Moon appears smaller than a quarter, we call it a crescent. When the Moon appears larger than a quarter, we call it gibbous. When the moon is getting bigger (phases New to Full) it is waxing. When it is getting smaller (phases Full to New) it is waning.  For example, if today the Moon were a waxing crescent, then we would only see a little piece of the half of the Moon that is reflecting sunlight. But tomorrow, that piece would be bigger. If today the Moon were a waning gibbous, then we would see a big piece of the half of the Moon that is reflecting sunlight. But tomorrow, that piece would be smaller.  There is an easy way to tell if a crescent Moon is growing or shrinking. If the crescent Moon looks like a "C" (C for collapsing!), it is shrinking.

material courtesy of the University of Michigan