This is a super-simple introduction to a complex topic. For fun, you can show this blog post to your knowledgeable musician friends, and ask them, "Which of these statements aren't true?"
All music is built from 12 basic notes (often called tones): A, A♯ (or B♭), B, C, C♯ (or D♭), D, D♯ (or E♭), E, F, F♯ (or G♭), G, G♯ (or A♭). This full set of 12 tones is sometimes called the chromatic scale.
Each piece of music is set in a key based on one of those twelve notes. The central note of the key is called the keynote or tonic, or simply [one]. When someone says, "This song is in E," they're telling you that E is the song's keynote.
In addition to its keynote, each piece of music has a mode. A mode is a flavor of tonality. The most famous modes are called major and minor. The combination of a keynote and a mode is sometimes called a key or a scale. "This song is in the key of A minor." "This melody uses the C major scale."
Twelve keynotes times two modes give us 12 x 2 = 24 different "keys." That's one for each hour of the day, if you like. J.S. Bach's The Well-Tempered Keyboard made a point of offering pieces in each of these 24 keys.
A scale is a set of seven tones chosen from the complete set of 12 tones. The seven chosen tones are said to be "in the key" or diatonic. The other five tones, if they appear in the piece of music at all, are said to be "not in the key," non-diatonic, or chromatic tones.
On a musical keyboard, this is easiest to see for the key of C major. The seven white keys in each octave are the diatonic tones in C major, the tones that are in the C major scale. The five black keys are chromatic tones if they appear in a piece set in C major.
The white keys on a keyboard also happen to be the diatonic tones of the key of A minor. Yes, A minor uses exactly the same set of seven tones as C major. Even though they share the same set of white-key tones, these two keys are distinctly different keys. The keynote falls in a different place in the set of tones, and that makes an enormous difference.
Though they are different keys, C major and A minor do share a special relationship. Because they use the same set of tones, it's quite easy to slip from the key of C major into the key of A minor, and vice versa. We say that A is the relative minor key of C major. C is the relative major key of A minor.
C major also has a special relationship with C minor. These two keys share the same keynote and four out of their seven tones. C minor is the parallel minor key of C major. C major is the parallel major key of C minor.
Every major key has its own relative minor and parallel minor. Every minor key has its own relative major and parallel major. If you can grasp these special relationships between closely related keys, you've got a big head start on the next chapter, when we go beyond three-chord rock and learn how to use dozens of different chords.
More technical details on keys, modes, and scales can be found at Wikipedia.
Tuesday is Beastly Fundamentals Day.