Sharp bettors are basically professional gamblers who earn a living by betting on sports. These types of bettors place high amounts of money on sports bets but they do so by placing smaller wagers at a variety of bookmakers in an attempt to go unnoticed. They also do this to gain maximum odds value.
As it is their job, "sharps" are extremely knowledgeable and forecast odds before bookmakers post the price. Sharps look for solid bets that will make them money - they won't just bet on an event just because it's available to bet on. If they are unsure, they will protect their bankroll and save the bet for another day. They will rarely stray from their strategy and rely heavily on statistics, research and detailed analysis rather than using their "gut" or their "instinct".
Most importantly, they only bet on markets that offer value, also referred to as a value bet.