I think the answer is as self explanatory as the question: He's interested in you on some level or another. It would take some more interaction to know if he was thinking friendly, romantically, sexually, etc., but there's obviously that initial enjoyment of you if he's smiling.
It could mean I like you, or it could mean I am just happy to see my friend, but there is no romance between us. Sometimes I smile at a girl I know to make her day feel better, because nobody else smiled at her or said hello.
Well it depend for me.. I smile at all my friends.. and crushes. If its a friend, I usually smile at them if we're passing through the hall at school or something but if your sitting in a class and you catch a guy's eye and he smiles then for me it's a: god, you're amazing sort of smile and usually for most guys they smile because they're thinking about being with you.
granted, some guys do undress girls with their eyes so watch out for them.
that wed like to try and talk to you, all you have to do is return the favor and smile back, don't do it, if you don't want us coming up to you though, nothing worse than a lead on (which is why natural flirts are so annoying)