A sharp is someone who bets lines early and then the line moves with lesser value than when the "sharp" bettor bet it.
A sharp also doesn't bet bad or stale lines.
A sharp has a bankroll and uses good money management, this way he can endure a bad streak that would end a square recreational bettors bankroll.
Finally a sharp is someone who profits from sports betting.
I promise you there isn't a lot of sharps in forumsville.