There are a lot more notes than c d e f g a b, they all have sharps (bit higher) and flats (bit lower)

When a song is in a key, we mean that it has a specific note as its base, and the other notes follow that notes scale.

A scale is a sequence of note distances, so no matter what note you play, the next note in a scale will always be the same distance. Think of it as a staircase that has a sound for each of its steps and when climbed up it always follows the same sound pattern.

A C major scale is played with all of the white tiles of the piano, from C to the next C. A song in C major will use those notes. Another scale, like G major, will need to use one black tile in order to keep the same pattern.

Basically, a scale is a pattern of sounds, and the key is just which note is being used as the first note in the pattern.