There are two kinds of criticism. Constructive criticism and destructive criticism. You can point out someone's mistake and give them a platform to improve or you can bash someone and totally discourage them.
For Example, ABC did the worst research paper in the world. The grammar/spelling mistakes, content wise wrong and not neat at all.
Now, the teacher can reply in two ways... One would be constructive criticism and other destructive.
"You are stupid, ABC!! Couldn't you have written a worse paper?" - Now, that's what I call destructive criticism or otherwise named bashing.
"Well ABC, there are a few ways I think you can improve i.e. grammar, spelling, and research. I know you can do much better, just give it a try!" - That would be constructive criticism. You have said what was wrong and could be changed whereas being polite and encouraging the person to work on it.