Use the adjective lesser to describe something that's smaller than or inferior to something else. A lesser singer might sound fine in the context of a large chorus, but a solo performance could be tricky.
A young car thief might be charged with a lesser crime because of his age — trespassing instead of larceny, for example. He's also likely to end up with a lesser punishment in that case. Sometimes people talk about "the lesser of two evils," by which they mean the less bad of two bad choices, or describe something as "lesser-known," like a rarely studied poet or an unacknowledged scientist.