Metallica put metal on a new level, and world took note. Metallica went on to sold more than 90 millions copies, which put them on 18th place of the RIAA list of top selling artists. They started as a trash metal band but moved, and refined its sound many times. They made trash metal into an art form. 
Metallica was heavier than the most metal bands out there and is most successful of the “big four” trash metal bands (Anthrax, Megadeath and Slayer in addition to the Metallica). Metallica created legions of fans, and creating them fast.
Metallica is often put categorized as a heavy metal, speed metal or hardcore. But they were and still are a trash metal band. Trash metal is a product of hard blues-rock music that started spreading 20 years before. The blues-rock genre started with Yardbirds, Cream, and Led Zeppelin and of course Jimi Hendrix. From here artist under the influence of hard blues-rock, such as AC/DC, Iron Maiden, Black Sabbath and Deep Purple, formalized heavy metal. Reinvented by The New Wave of British Heavy Metal (NWOBHM) around early 1980s metal was ready for a new sound. And in this point in time Metallica got down to business of creating heavy metal heavier and through than going to its new evolution, trash metal.
New sound was heavy, fast and complex. Metal was at its best and Metallica was in the middle of it. As band released first demos in 1982. world was preoccupied by the fancy guitar work such as Eddie Van Halen, Randy Rhoads, Motorhead, Def Lepard. They were influenced by the NWOBHM, and then ventured on to unleash their new sound and style. All the leather and chains Van Halen copycats were out of business. Metallica was reformatting music to their sound and style.
Metallica is a mix of socially aware lyrics, heavy and complex rhythm riffs, and ear catching, technically challenging solos. The band is also well known for its ambitions arrangements, new modulations and multiple tempos. Metallica is purely guitar driven. Always was and always will be.
Troubles around them started early as their lead guitarist was too aggressive, too drunk and too drugged to control. David Mustaine was informed that he was getting to out of hand and that he must leave (later he created very successful Megadeath). His place took Kirk Hammett, former pupil of Joe Satriani. Kirk brought his own style and sound to the band and managed to successfully combine his playing style with James Hatfield’s famous rhythm style.
Next I’ll release a how to guide on style of Metallica.
