The 1990s was a very transitional period for metal, as the decade was split between two eras of the genre. The first half specified in the old school 80s style, but the latter was littered with the next level, the hip-hop, industrial-esque style of nu metal that would take the genre into the mainstream.
As such, this era was prosperous for a great many bands, whether they were upstarts, or existing veterans, and the resulting albums were heavy, unique, and highly experimental.
The era helped to bridge the gap between social groups, and offered a wide range of sub-genres for metal fans to sample. Rap metal, industrial metal, nu metal, and metalcore all got their big starts in the eyes of the genre's faithful, as did a great number of artists we know and love today.
Some would improve on their existing style, and some would completely reinvent themselves to fit a new era.
Love 90s metal or hate it, it is undeniable that the evolution of the genre during this decade is fascinating, and some bands took the opportunity to break the mould of what was expected, producing the best work of their careers.