THRASH METAL is a subgenre of heavy metal that is characterized most typically by its fast tempo and aggression. Thrash metal songs typically use fast percussive beats and fast, low-register guitar riffs, overlaid with shredding-style lead work. Lyrically, thrash metal songs often deal with social issues and reproach for The Establishment, often using direct and denunciatory language, an approach which partially overlaps with the hardcore genre.

Thrash metal’s “Big Four”, the four bands widely regarded as the genre’s most successful and influential acts, are Metallica, SLAYER, Megadeth, and Anthrax due to their status as pioneers of the genre in the 1980s. Some common characteristics of thrash metal are fast guitar riffs with aggressive picking styles and fast guitar solos, and extensive use of two bass drums as opposed to the conventional use of only one, typical of most rock music.
In 1981, a American thrash metal band by the name of Leather Charm wrote a song entitled “Hit the Lights”. Leather Charm soon disbanded and the band’s primary songwriter, vocalist/rhythm guitarist James Hetfield met drummer Lars Ulrich through a classified ad. Together, James and Lars formed Metallica, the first of the “Big Four” thrash bands, with lead guitarist Dave Mustaine, who would later form Megadeth, another of the “Big Four” originators of thrash, and bassist Ron McGovney. Metallica later relocated to the San Francisco Bay Area. McGovney was replaced with Cliff Burton, and Mustaine was later replaced with Kirk Hammett. The band released “Hit the Lights” on their first studio album, Kill ‘Em All, in July 25, 1983.
Another “Big Four” thrash band formed in America in 1981, when guitarists Jeff Hanneman and Kerry King met while auditioning for the same band and subsequently decided to form a band of their own. Hanneman and King recruited vocalist/bassist Tom Araya, a former respiratory therapist, and drummer Dave Lombardo, a pizza delivery driver, and Slayer was formed. Slayer was discovered by Metal Blade Records executive Brian Slagel while performing Iron Maiden’s “Phantom of the Opera” at a show, and were promptly signed to the label. In December 1983, less than six months after the release of Kill ‘Em All, Slayer put out their debut album, Show No Mercy. Here is a list of top 10 American thrash metal bands:

(SOURCE” WIKI – http://en.wikipedia.org/wiki/Thrash_metal)