What's the difference between Too and Very?

Adding one grammar aside to the excellent commentaries given above.
Both of them go with adjectives:
Very and Too - emphasizes (intensifies) the word that comes after (more often than not - adjectives, but sometimes an adverbs.).
As pointed out "too" rings a negative sense to the following word, and "very" usually doesn't connotes a negative sense.

In the sentence "he drives very fast." it could be both, in a neutral sense or a negative one, though (depending on your tastes, whether you like speed or not.)

Complementando a sugestão de Mateus:

Very = muito
Too = excessivo, demasiado (comumente de modo negativo)