Well, it depends. If you know you were looking bad and being rude or annoying or weird he is probaly looking at you because you are weird to him. But!! If you look fine and are being nice, it probaly means the guy is checking you out and is interested!
Girls, when a guy does this it might mean he likes you but he might be sizing you up... What I mean by this that put simply he thinks your cute. Also if he does this nonstop He absolutely does like you or if hes staring at lets say your butt... he's a pervert.
It may very well mean that a drugged guy is looking at you
That he wants to "get to know you better."
Facebook.
They have the same interests
he is starstruck
Nothing. He is a nice guy.
it means she likes him
Not necessarily.
He is simply a guy! guys do that unfortunately!!
It means he's trying to subtly tell you that he loves his girlfriend, and any advance you make is a sign of disrespect to him and her.
If a guy looks at a girl when she is not looking, that does not always neccessarily mean he likes you! Hope this helps! =)