When a song consists of notes from a particular scale, it is said to be written in the key which has the same notes as that scale. For example, if a song contains mostly notes from the C major scale, it is said to be in the key of C major. The songs you have played in this book that start with a C chord written above the first bar of music are in the key of C major.
The following two songs are in the key of C major.
In any particular key, certain chords are more common than others, and after a while you will become familiar with the chords that belong to each key.
Certain keys are easier for guitarists to play in and you should learn how to transpose (change the key of a song) so you can change a song that is in a difficult key (contains lots of sharps and flats or difficult chord shapes for a beginner to play) into an easier key.
The most common chords in the key of C major are;
For more information on transposing and chords see Progressive Guitar Method: Rhythm .