Not all time signatures are the same.  While we are most familiar with simple time such as 4/4 or 3/4, other time signature such as 6/8 exist.  When you set the tempo of a song and play it in 6/8 time, you may notice that it plays much faster than other tempos.  This is because it's a compound time signature.

When you play in 6/8 time, you are actually playing 2 dotted quarter notes per measure. Since tempo is based on the duration of a quarter, and because we are essentially fitting three quarter notes into the space where there is normally 2, the duration of each eighth note is reduced resulting in a tempo that seems apparently faster, but still keeps the tempo basis on the quarter note.  Another way to explain this is 6/8 is like playing a song in 2/4, but playing a triplet (three notes) for every quarter note.  This makes the tempo sound about 50% faster.