Collection of sharp and flat symbols at the beginning of a musical staff, indicating the tonality of the music written on that staff.

For the non-musical people out there: listen to your average pop song. Usually there's a certain note or set of notes (called a chord) that feels like home -- a good landing place for a note sequence. That "home" note is the base of the tonality or key. This key is represented in musical notation by a set of markings indicating modifications, either raising the note a half-step (sharp) or lowering it one half-step (flat), to any notes written on a particular line or space.

Each key signature represents two possible keys, a major and a minor key. The most simplistic way to explain this is that major keys sound happy, and minor keys sound sad. As with most simplistic explanations, though, that doesn't do justice to the concept. The minor key for a given major key signature starts on the perfect sixth of that major key.

C major is the key requiring no modifications (accidentals), and hence requires no key signature (null key signature?).