| Am Dm | G C |
One of the most popular forms of reading music are chord charts. Chord charts ( or lead sheets) are used by various types of musicians, from jazz pianist to worship leaders. These charts are a subset of standard sheet music. They combine indivdual notes into what's called a chord symbol. For example, the symbol "C" indicates to play the "C chord". The "C chord" is comprised of the notes C-E-G.
| Dm G | C |
These are called bar lines. They separate the chords into groups called measures. In the example above, measure one contains "Dm and G" chords. Measure two contains the "C" chord. Generally, a measure is equal to four beats (not always). In measure one, their are two chords in measure one. Consequently, Dm gets played for 2 beats and G gets played for 2 beats. However, measure two has one chord, the C chord. It gets played or held for 4 beats.