The Phases of the Moon. Click on image for full size (15K GIF)

For example, if today the Moon were a waxing crescent, then tomorrow the crescent shape would continue to grow larger, approaching first quarter. After first quarter, the Moon would be a waxing gibbous, and continue growing until it reached full. The Moon would then begin to shrink, becoming first a waning gibbous and eventually reaching third quarter. Following third quarter it becomes a waning crescent, and continues to shrink until it becomes invisible at new Moon.

Just in case you can't remember all of this, there are a few handy ways to recognize whether the Moon is growing or shrinking. A crescent moon which looks like a "C" is shrinking (C for collapsing!). If it looks like a "D", then it is growing. This is true for a gibbous Moon as well, but it's a bit trickier to picture. If the edge of the Moon (the real edge of the Moon, not the edge of night on the Moon) is curved like a "C", the gibbous Moon is shrinking. Another way to think of it is that the Moon always grows or shrinks from the right to the left.