A Sub-Genre of Metal made popular during the 80's originally by bands such as Motorhead and Venom. Metallica then took the style and made it into a more popular and prominent style. Other bands through the Thrash ages are Overkill, Exodus, Testament, Megadeth, Sepultura, Vio-Lence, Dark Angel, Death Angel, Kreator and so forth.
Basically Thrash is a fast paced Metal style that usually includes lots of Palm Muting on the top E string and mega fast solo's.
"Did you see Overkill play last night"
"Yeah, they were Thrashy as fuck!"