Quick synopsis of the differences:
Lava and Ultra produce different-looking snakes.
Lava and Ultra are incompatible with another (you can't breed an ultra to a lava and expect to get hypos)
Lava is a simple recessive. Ultra is co-dominant to Amel.
For more info, do a search on lavas, ultramels, and ultra. Lava has also existed under the name 'Transparent hypo'.
-Kat