Thrash metal is characterized by heavy and jerky rhythmics and riffs. Old METALLICA albums were thrash metal. Being fast is not enough to be thrash. For example, bands like HELLOWEEN, GAMMA RAY, STRATOVARIUS and others play songs which are sometimes very fast (not as fast as KREATOR, but still fast) but they don’t play thrash. On the other hand, thrash metal is not inevitably fast. Some KREATOR songs are not that fast : for example, “Violent Revolution” is not really fast, but it’s a thrash metal song. In fact, thrash metal is, to me, a kind of metal where rhythmics are heavier and jerkier than in heavy metal. That’s why METALLICA and KREATOR are thrash metal (but nowadays, METALLICA plays more a kind of heavy thrash than real thrash).
People often say that there are two kinds of thrash metal : “simple” thrash metal (for instance, METALLICA and MEGADETH) and extreme thrash metal (KREATOR, SLAYER…).