Apocalyptica is a Finnish band consisting of three classically-trained cellists and a drummer:

Eicca Toppinen - cello
Perttu Kivilaakso - cello
Paavo Lötjönen - cello
Mikko Sirén - drums

Former Members:

Antero Manninen – cello (1996–1999, occasionally plays live)
Max Lilja – cello (1996–2002)

Their specialty is heavy metal music on cellos, though they also play classical music.

They started by playing covers of Metallica, but once the concept became popular they wrote their own original works, maintaining the thrash metal influences on their albums Inquisition Symphony (mainly covers as well) and Cult.
Since Reflections, they have added drums to their recordings and live performances.

Covered bands and composers:
Metallica
Edvard Grieg
Pantera
Rammstein
Sepultura
Slayer
Faith No More
Savatage

Collaborations with:
OOMPH!
Dave Lombardo (Slayer)
Lauri Ylönen (The Rasmus)
Linda Sundblad (Lambretta)
Matthias Sayer (Farmer Boys)
Nina Hagen
Sandra Nasic (Guano Apes)
Ville Valo (HIM)
Marta Jandova (Die Happy)
Emmanuelle Monet, aka Manu (Dolly)
Şebnem Ferah
Sepultura
Rammstein (Played Ohne Dich and Mein herz Brennt live with in 2005 and also made a remix of Benzin)
Tool
Max Cavalera (Soulfly)
Matt Tuck (Bullet For My Valentine)
Waltari (Space Avenue)
Corey Taylor (Stone Sour, Slipknot)