What is sonata form?

Sonata form is formed of an exposition, development section and a recapitulation. The exposition comprises of a first subject, a bridge passage, a second subject and a codetta. The development section builds upon the musical material in these subjects in passages called episodes. The recapitulation is then a repeat of the exposition but with the second subject now in the tonic key, as opposed to its (usually) dominant version first seen in the exposition. There is also usually a coda after the exposition which uses musical material from the subjects to end the piece.