**[Birkhoff's HSP Theorem](http://en.wikipedia.org/wiki/Birkhoff%27s_HSP_theorem)**

I liked the preservation theorems personally.  Birkhoff's HSP theorem which identifies model classes of certain equational theories as being those classes (known in universal algebra as varieties) closed under homomorphisms (*H*), (iso to) subalgebras (*S*), and (iso to) products (*P*) is the one I remember most easily.  There are other versions (e.g. for quasivarieties) as well.  I hope this is magical enough for you.

Gerhard "Ask Me About System Design" Paseman, 2011.08.29