What would a key signature tell you in music?

A key signature tells you if there are any sharps and/or flats in the song/piece. A sharp will be represented by # and a flat by a b.

This is an example of key signature.

In this example, the notes that are sharp are F and C (I'm not sure about the second one...don't tell anybody though...O.O) This is because the sharp is in/on the spot where any type of rhythm goes.

  If you mean everything, and not just the key signature, there is the time signature as well. There is a number over 4 and it tells you how many beats are in a measure. The number can be four over four (also known as common time), a number lower than four over four, or a number higher than four over four.

  There is also bass clef and treble clef, but I'm not very familiar with bass clef, so I don't think I can tell you much about it. But I believe if there is a change from bass to treble (vice versa) there is a change in key. However, I'm not sure.

Hope this helped! If I forgot anything, just comment, and I'll do my best to explain.