In its most common usage the word hater is essentially used to describe some one who in any way dislikes or criticizes someone who is better looking than them. The new meaning of the word has recently come into play due to the discontent many attractive people have in regards to how the rest of the world deals with their unsavory behavior. Also while there is no concrete data to support this, but it would appear the term is most commonly used by females.
One guy: Some ugly fuck was bitchin at me like crazy just cuz i parked my car in front of his driveway. What the fuck?!
Other girl: Dont worry sweety hes just hating you know. Hes a hater