"I like you," simple as it may sound, has several meanings, all depending on context;
1) When you give your boss a good scolding in front of the entire office personnel, people might come up to you and say: "I (really) like you." And by that they would just mean: "I like your style; I love your way of doing things."
2) Most common usage of "I like you." is simply to express that you are very fond of this person. Again, context is everything. When a 6-year-old boy tells the girl next door that he really likes her, it probably means he's in love with her (or, at least, what he thinks is love). When your 23-year old girlfriend tells you "I really like you." (especially when preceded by the dreaded words: "We need to talk."), then it means your relationship is over. :)