Form
The structure of a composition, the frame upon which it is constructed. Form is based upon repetition, contrast, and variation. Generally, many pop songs fall under a AABA pattern where each of the A sections are repetitions of the theme (8, 12 or 16 measures) and the B section or "bridge" is the development (8 or more measures). A theme is at the start AND at the end. Surrounding this form, there may often be an intro and outro.
Composition
1. Any musical work or production.
2. The act of compiling and inventing a piece of music.
Repeat
In some compositions the composer will call for the repeat of a certain notes (beats), measures or sections. These notes, measures or sections are indicated by repeat signs that are specific to each type of repeat.
Variation
A deviation from a theme that uses the same bass pattern or harmonic progression that the theme used, and usually having the same number of measures as the theme. Generally, a variation is played after a theme with the variation being slightly more ornate; in several cases there are many variations upon a single theme. Variations are often used as accompaniments to songs that are several verses long, or as dance music. In both cases, the repeating structure is beneficial to the nature of the function.