input gain vs output gain

[read]

can you explain me what the difference is?

they both raise volume to my ears, but i don't know which one i should use its just guesswork

[DVDdoug]

For a compressor?

The input gain is applied before compression and the output gain after compression.

Increasing the input gain will push more stuff over the threshold giving you more "compression effect".

More output gain simply increases the level without affecting the compression that's already done.

Make-up gain would be on the output side.