A quick lesson in theory; in music there is something called a pitch. A pitch, corresponding to a letter, is a frequency that the human ear can recognize, which is also consistent through something called Octaves. There are seven unique pitches, ranging from A-G. When these pitches are used, they are referred to as notes. The space between notes, so going from an A to a B, is called an interval. When you stack a series of notes in a specific way, with a strict rule of how the intervals work, you get what is called a chord. An example of a chord is a C major chord, which requires the C note, the E note, and the G note. The way the chord feels when played is what dictates why we call chords Major or Minor. The most important part in theory is the pattern or route a series of chords takes. This is because of two points of interest; tension and resolution. Tension is when a chord is unpleasing to the ears, and demands a change to a different sound. Resolution is only possible after tension, and is when the chord is what the ear wants to hear. Most charts (songs) have what is called a Chord Structure, which is a set pattern of chords, going from the starting chord, to a build up, followed by tension, and resolving back to the starting chord, which generally acts as resolution.