A typical number chart may look something like this:
Every number is a measure. This particular chart is only 8 measures.
Dashes indicate minor chords. All other chords are assumed Major. Slashes show that a bass note has been altered.
If this song IN THE KEY OF C were written using chord symbols it would look like this:
If we changed the above number chart to THE KEY OF D it would look like this:
We would have had to re-write the chord symbol chart in order to change keys, but the number chart stays the same!