Become a composer! Part 2: Learn Your Instruments

[MetalRenard]

Classical music is harder to understand because it is so much more complex than other genres when it comes to understanding the roles of each individual instrument in the whole.

We'll take a step back and look at rock music first since it is so much more well defined and each instrument has a precise, easily discernible role.
The drums take care of rhythm and do lots of the work for transitions. The bass can be independent but quite often its place is to hold the whole thing together. Without a bass guitar the drums and guitar wouldn't fit together.

Now for your answer, "Backing" would represent what the rhythm guitars do during a solo.
Example: http://metalrenardmusic.bandcamp.com/track/a-lifes-work
For almost this entire piece there is a lead melody and separate backing. Do you see how their roles differ?