A duster is somebody who is worthless; a loser. Somebody who isn't good at anything.
Usually used by athletes to describe someone who is horrible or worthless at a sport. Usually the worst player on their team, and as a result all they do is sit around and collect dust, hence the term "duster".
Jake is such a duster. In gym class all he does is stand in the corner while we play dodge ball.
A tool is someone who can't think for themselves. Someone who is easily taken advantage of, picked on and made a fool out of. Someone who is being used and doesn't even realize it. Someone who tries too hard to please others. Someone who can't defend themselves.
These acts give people the impression that the "tool" doesn't have a brain or any common sense and as a result all they are good for is being used, like a tool(hence the insult).
Being called a tool is NOT a compliment.
"Eric is such a tool. He doesn't even realize that all these people are just making fun of him."
"Yeah. I really feel bad for that kid."