Criticism usually means saying what is wrong with something. Critics write criticisms in their reviews. They may say good or bad things about what they are reviewing.
The word 'criticism' is often used in general life when someone 'criticizes' someone else (says that what they are doing or have done is not good'). Criticism does not have to be unkind, it can be given in a helpful way. For example, an adjudicator in a music competition may give helpful criticism ('constructive criticism') to the performers so that they can learn from the experience and make their playing better.
The adjective critical usually means criticism that can hurt. 'Critical' also has a different meaning: Someone who is 'critically injured' is in danger of dying from their injuries. A 'critical moment' may be a moment which will show whether something is going to work or not.