Orchestrock is (to put it simply) a combination of Rock instruments (heavy guitar, drums ect ect) played with a live orchestra.
The occurance of this type of music has been rising as of late, and a first example that would come to mind would (at least) be Nobou Uematsu's "Black Winged Angel" with his own band "The Black Mages" for the CGI movie "FFVII- Advent Children".
Hey man, this Orchestrock is amazing!
What the fuck are you?, insane???, Slayer is obviously better :|