To generalize:
Speed Metal is typically more technical & melodic. It includes melodious vocal lines and riff structures (lots of reverse power chords and intricate string picking). The solos all follow melody lines too (see Marty Friedman). You'll also get sweeping appregios, or bluesy emotional playing -- in other words, there are lots of hooks in the songs. See: Accept (who defined the term), through to Megadeth and Annihilator.
Thrash is all out devastation & terror, slightly more simplistic riff structures (regular power chords being 'thrashed' out). The solos are blistering and mostly void of any scale structure (see Kerry King), and the vocal lines are 'barked' out. The obvious next evolutionary step from these bands is death metal. There's not as much crowd singing along I guess. See: Slayer, Dark Angel, Lethal, Sepultura.
Bands that might sit in the middle: Metallica & Testament. Indeed, Metallica always denied the thrash labelling.
Not strict of course, just an opinion.