"Theme and variation" is a form of music in which the same theme or pattern returns in slightly different variations of the original. The theme is a pattern that is easily recognized and used to make the piece cohesive. Variations focus on ideas of repetition and contrast. The form can be written as "A-A1-A2-A3, etc," where A is the theme.
A variation occurs when a particular section returns but is not exactly the same as the original or previous iterations. A successful variation ensures that the theme is discernible. About.com Music Education explains that techniques typically used for theme and variation include instrumental, harmonic, rhythmic, style, tonal and ornamentation.