A Lead Sheets is sheet music where the melody is written, and there are chords above each bar.
This means that you can play the chords, melody and sing the melody as well. You can also use it as sheet music for a band, as the piano, guitarist and bassist all have the chords and the vocalist has the melody and lyrics.
In Jazz, lead sheets are very common. There is a famous book called The Real Book, that is full of jazz standards in lead sheet form.
On the left, is a lead sheet of My Shining Hour from The Real Book. It is an instrumental lead sheet. If you had a jazz quartet, The Lead player (typically saxophone, trombone or trumpet) would play the notes, and the pianist would play the full chords, the bassist would usually play the root note of the chord, and the drummer has the structure and the style/tempo marking of medium ballad.
On the right, there is a lead sheet of Valerie. It is similar to the other lead sheet, but it has lyrics because it is written to be sung by a vocalist.
Lead sheets are very useful, especially in Jazz, as you can play the head with the written notes and then solo with the chords. However, a lead sheet doesn't have any repeats in so you may have to change bits as you go, and you will have to decide in the band what you are going to do at the endÂ