What's the difference between a sidereal and synodic month?

Answer 1

http://en.wikipedia.org/wiki/Sidereal_month#sidereal_month

By the time the moon has travelled around the earth, the earth has also moved in it's orbit around the sun, so from earth's point-of-view, the moon will line up with certain stars (constellations) a few days sooner than the synodic month (because the earth is "ahead", looking "behind" toward the moon/background stars.

It takes another ~2 days for the moon to "catch up" with the earth's orbit around the sun, to complete its phases.

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Answer 2

Because sidereal monthes depends on the moon's revolution around the earth which has nothing to do with synodic monthes which depend on the earthes revolution around the sun.