Some would group the two genres together as one, others would claim that one evolved into the other but are still essentially different.
Both genres of metal offer fast paced, aggressive, sometimes quite erratic instrumentation. Some claim that where Speed metal differs is in the presence of a little more melody.
There are some very good points of reference for this kind of music right here at MB. I'm sure Jackhammer will pop his head in at some point and can hopefully link you all to his thrash thread and compilation.
For now here are a few good starting points.
Speed metal RYM
Speed metal Wiki
Thrash metal RYM
Thrash metal Wiki