A Nice Guy is a guy who is friendly, kind, and understanding toward girls with the underlying condition that they must fall in love with him because he is the only friendly, kind and understanding man they've ever met. Nice Guys think that acting like a decent person makes them special and entitled to women's romantic affections and sex.
They believe that women have no minds or emotions of their own, and that they exist solely to reciprocate the affection of whichever man 'deserves' them. They will often try to guilt women into going out with and having sex with them.
If and when a woman rejects a Nice Guy, he will whine and moan and complain that girls are stupid and don't know what's good for them, and only like jerks that won't respect them or their feelings. Nice Guys invariably fail to see the irony in this.
Not to be confused with an actual nice guy (no capital letters!), a guy who is kind and decent without the underlying expectation that women are obligated to sleep with him or return whatever romantic affections he might have toward them.
Nice Guy: I'm so sorry that happened to you, here, go ahead and cry on my shoulder.
Girl: Thanks, you've really made me feel better. You're such a good friend.
Nice Guy: Great, now let me take you out to dinner and have a romantic evening ended with sex and marriage.
Girl: Sorry, I'm not really interested in you that way.
Nice Guy: Fuck you, you cold bitch. You're so stupid, why can't you see that I deserve you? After all the nice things I've done for you, you owe it to me to fall in love with me. Fine, I guess I just have to keep harrassing you and trying to guilt you into going out with and having sex with me.