Okay, here are the basic parts of music theory which I can remember:
1) There are only 12 notes (in Western music). Those notes are A, A sharp, B, C, C sharp, D, D sharp, E, F, F sharp, G, and G sharp. Notice that all the notes have sharps except B and E. Another way of looking at it is that all notes have flats (A sharp=B flat, F sharp=G flat, etc.) except C and F.
2) The 12 notes are divided up into whole steps and half steps as a result. In other words, A and B are a "whole step" apart -- there is another note in between them. In contrast, B and C are a "half step" apart -- there is nothing in between them.
3) Consequently, going up and down various musical scales is simply playing a progression of notes organized in whole- and half-step intervals. For my purposes, the only musical scales I discuss in this website are the major and minor scales. In formal music theory these scales have Greek names, but for a layman this is just confusing.
4) Music is written on a staff, in which each line or space represents a different note in alphabetical order going from the bottom to the top. A full musical staff (such as the kind that a pianist would use) consists of a treble clef and a bass clef -- basically two different sets of lines for two different registers. The music for the electric bass, for example, would be written in the bass clef, while guitar music would be interpreted on the treble clef. While it's good to know about how music is written, a person can understand music perfectly and still be musically illiterate -- in fact, I would bet that most musicians are musically illiterate. Jimi Hendrix and Paul McCartney are excellent examples of musicians who could not read a note.
5) As a very broad, general rule, music in a major key comes across as being stronger and happier. Major key music somehow seems more "confident." In contrast, minor key music seems brooding and sad. Minor key music gives off an air of uncertainty. To offer a useful example, compare a song like the Ramones' "Blitzkrieg Bop" to Neil Diamond's song "Solitary Man." The former is a tune that you would expect to hear at a college football game it's an exciting song with visceral tones. The latter invites introspection, it seems uneasy and pensive.
6) This site won't delve into the intricacies of staff notation much, since it is geared to be an introduction, but the number of sharps (#) or flats (b) written on a staff determines a song's key signature. For example, a staff written for a song in the key of C major does not contain any of these accidentals. The seven notes in that scale are natural -- they are not sharped or flatted. Similarly, the staff for an A minor key has no sharps or flats. The difference between these two keys? They simply start at different places -- A is the sixth note in C major, C is the third note in A minor. All the key signatures can be written on the staff, simply by adding the requisite number of accidentals to the front of the notation.