A strong and extremely complicated positive emotion that a straight male feels towards another male. It is by no means homosexual, although it could possibly involve some degree of sexual attraction. This is not the type of sexual attraction that one would actually pursue; it is merely a complex sort of desire out of "fantasy" that is driven by jealousy and/or great admiration.
A 'man crush' can basically be described as not a homosexual lust, but a human lust.
You know that one football player who's a senior with the perfect skin, the perfect smile, the dark short hair, the soothing voice, the easygoing and fun attitude, and the perfectly tan and fit and muscular body that has all the girls on campus going crazy? I mean, c'mon, what guy DOESN'T have a man crush on him? He's perfect... It's so unfair... LOL