Mark Zimmerman wrote:

>can anyone please define this term so a simpleton like me can understand
>them.

I'll draw this very quickly.  I hope this may help as a preliminary
gossip (I mean, before reading good bibliography:-).  "Sonata" is a type
of composition, involving 1 to 4 instruments of diverse combinations
(the baroque trio sonata, for example has a "continuo" performed by
harpsichord/organ and a bass instrument).  "Sonata-form" instead, is
not a type of composition, but a formal scheme.  A formal scheme is a
sort of thematic and tonal map of a particular movement (not necessarily
of the whole composition).  There are other formal schemes.  Usually,
the "sonata-form" is the scheme of the first movement of a classical -
romantic composition (a sonata, a concerto or a symphony).  The classic
sonata-form took its shape approx.  towards the middle 18 century.  The
scheme involves 2 themes, exposed and developed in 3 sections.  Its most
simple formulation could be this:

A (exposition of themes 1 and 2) - B (development of one or both themes) -
A' (recapitulation of both themes + coda (a short "ending" section)

If the theme 1 is in major mode, the theme 2 is usually in the dominant key
(for example: G major - D major), but if theme 1 is in minor mode, theme
2 is usually in the relative major key (for example: c minor- Eb major).
Of course, this is just a simplification of the scheme (in fact, it's a bit
more complicated than this).  You'll find a lot of variations and changes
of this model at Haydn, Mozart and Beethoven's works (in fact "models" are
often a matter of statistics).

Pablo Massa
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