A womanizer is a man who has sexual relationships with lots of different women, treating them as conquests rather than as people he cares about. The word says something about his pattern of behaviour over years, not about one affair, and it is always disapproving — you are saying he uses women. Notice how it differs from words nearby: a 'playboy' is rich and enjoys himself and the word sounds glamorous; a 'flirt' only talks and jokes, without necessarily doing anything; 'cheat' or 'cheater' is about breaking a promise to one partner. 'Womanizer' sits in the middle — it is about quantity and about not caring. The tone is disapproving but not the strongest word English has; it can even sound slightly indulgent, the tired way people talk about an old actor or politician ('he was a terrible womanizer'), which is why some writers now avoid it and use harder, clearer language when the behaviour was actually abusive. It is a normal word for reading and for talking about people in the news; British writing may spell it 'womaniser'. There is no common word used about a woman in the same way — English has never made one, and that gap tells you something about how the language grew.