Tro = "too" or "overly"
Tre = "very"
Both generally live in the vicinity of adjectives.
La tre bela pantalono estas tro mallonga. = The very lovely pants are too short.
Tiu tre inteligenta viro estas tro afabla. = That very intelligent man is too kind.
Tro multe, tro ofte = Too much, too often; as opposed to, Tre multe, tre ofte. = Very much, very often. (Both of which have been said about my commenting here.)
Then there's Tra = through.