How to read lead sheets
Lead sheets typically contain only the melody line, chord symbols and the partial lyrics of a song (if the song contains lyrics). The given information is reduced to the minimum that is necessary in order to be able to play the song correctly and leaves a lot of space for individual interpretation. The song can be played in different styles, at different tempos, it can be reharmonized and the melody can be variated rhythmically and melodically.
The chord symbols
A chord symbol contains information about the basic major, minor, diminished or augmented "root-chord", the additional seventh of the chord and about optional add-on notes that are frequently used in jazz music to brighten the harmony up with additional colours.
The first letter contains the information about the root note of the chord and is followed by nothing or "maj" (major) in case of a major chord or "m" or "-" (minor) in case of a minor chord. Sometimes also an upper case letter is used for major and a lower case letter for minor.
The next important thing to look are the numbers. In case of "7" you have to play the seventh note of the root notes´ minor scale. Only the combination of "major" and "7" "maj7" means that the seventh note of the root notes´ major scale is required. A higher number like "9", "11" or "13" means that this certain note of the root notes´ major scale and also the seventh have to be added, a major in front of a higher number means the same but with the major seventh implicated. A "11" and a "13" chord also implicate the 9th.
Sometimes the fifth of a chord is also variated and augmented "aug" or "+" or diminished "dim". Alterations of notes are often written in brackets e.g. (#5), (b5), (#9), (b13).
Those were just the most commonly used ways to write chord symbols, there are a lot more that are likely to appear in Realbooks, libraries etc.: