Womanizer definition can be different things to different people. To some guys, being a womanizer is the be all, end all of existence, while to others, it may not be an honorable thing. Generally, womanizer is defined as a man who pursues multiple women, preferring casual relationships over long-term, committed ones. For some, being a womanizer is something to aspire to, while other people, both men and women, see womanizers as commitment-phobic players.
A womanizer knows how to treat women right, for a short amount of time, anyway. While a man who is looking for a relationship with one woman will do things to impress that one woman, a womanizer will do nice, respectful or sweet things for a number of women, often at the same time.
One way to define a womanizer is a guy who has only male friends. A womanizer usually has a close group of men as friends, but no real female friends. This is most likely because he has alienated most women he's come in contact with by constantly pursuing the next woman he sees.
In some cases, the definition of womanizer is not positive. While both men and women occasionally want sex with no commitments, womanizers are after it constantly and may say or do whatever it takes to get the next woman in bed. An example of a womanizer in popular culture is Barney Stinson on the sitcom "How I Met Your Mother." Although he is an extreme case, Barney is constantly concocting plans and schemes to have sex with women, only to leave them behind the next day.
You can spot a womanizer in several ways. Look for the guy who seems to care a bit too much about his appearance. If you find him on a social networking site, usually a number of different women will have posted on his page. Another way to spot a womanizer is to look for the guy who always has a different girl on his arm and seems a little unsure of who exactly the girl is.