Criticizing puts more attention on the thing/person being criticized. Complaining puts more attention on the person complaining. When you criticize something, you’re saying “Here’s what’s wrong with it/you,” but when you complain you’re saying “I’m not happy about this.” It may just be semantics, but there is a difference in the mindset and attitude.

Complaining focuses on a person’s behavior. Criticism focuses on their character. “I don’t like it when you leave your towel on the bathroom floor” is a complaint. “Are you too lazy to pick up your towel?” is a criticism.
When we voice a complaint, we identify a specific behavior that we’d like someone to change. When we use criticism, we make it about a person’s self.
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When you want to let your spouse know that something is bothering you, you can start with, “I have a complaint.” When you hear those words from your spouse, it is a cue to take a deep breathe, listen, not to get defensive and to hear their perspective. See it as an opportunity to learn what can enhance your relationship.