Ps-Don't go OD on the stalker thing. Not only is it a turnoff--it's also pretty darn creppy...
The "puppy love" or "OMGSOHOTIWANNADOHIM/HER" kind of crush. The kind when you see someone attractive or cute, you'll basically love them. Most commonly associated with adults such as teachers and celebrities (Brad Pitt and Jessica Alba being common ones). 99.9% of the time, these crushes aren't serious, just a silly little infatuation. Some girls/guys may keep posters of their celebrity crush, masturbate to them or really just like them for the sake of their friends.
The REAL kind of crush. When you have a good-looking or nice/funny/generous/etc friend of the opposite sex (or same sex if that's your thing) or an acquaintance/stranger such as "the guy/girl in my class". In the case of a stranger or acquintance, it COULD be Meaning 1, but if you find yourself a bit more.. obsessed, then it's Meaning 2.
These crushes are painful to experience, but also exciting and addictive to experience. Even if you feel you might not ever get to go out with your crush, you still enjoy the adrenaline and excitement of having a crush; having him/her talk to you, seeing him/her around school/college/university, and reading his/her Facebook or MySpace.
Crushes tend to have a 50/50 success rate. Some fail, some succeed, so keep optimistic!
Karmia has a crush on Matt Damon ever after watching his Bourne films. She has his autograph and has a poster of him upon her wall. She knows she'll never go out with, he's a celebrity and probably forgot who she was by now, but she still admires his looks. Just like Jamie has a hot Physics teacher who he'd love to do, of course, she's 35 and married with 2 children, he's only 17. So no chance, but he enjoys playing with his imagination.
Meaning 2 Examples:
Steph has a crush on a guy that was in her Geography class. She thinks he's hot initially, BUT also finds he's a nice and humorous person, she speaks to him every now and again, but he has no idea she's crushing on him. Luckily, he is single, and she is speaking to him more and will eventually go out with him. Of course, bad news for Ben who has a crush on his female best friend, Olivia, because he's just found out she's going out with a boy named Daniel.