Can someone tell me how to clean my air ducts?

One thing I am particularly interested in is the return-air grill cover. This is not where the air conditioning or heat blows out. This is a 2.5' x 2' cover on the wall. I took this cover off and looked inside. The side, top, and bottom on the inside, was covered with well, it looks like some type of padding. To help you better in answering my question, this stuff also looks like different colored lint. Is this stuff suppose to be there, or is this what I want to clean out? The stuff on the inside is about an inch and a half thick on all sides. Maybe this is insulation, I don't know. That is why I am asking for help.