Public betting percentages show what portion of betting tickets (and sometimes dollars) are placed on each side of a wager. This data is published by some books and tracking sites and is used as a signal to distinguish public ("square") money from sharp ("wiseguy") action.
Ticket percentage vs. dollar percentage: Ticket % counts individual bets, which skews toward recreational bettors who make many small bets. Dollar % counts the dollar amount, which better reflects where large bettors (sharps) put their money. When the two diverge sharply, it signals that small bettors and large bettors disagree.
Square tendencies: The public systematically overweights favorites, popular teams, primetime games, and overs. Books know this and may shade lines toward the public side to attract balanced action — sometimes creating value on the square side in the opposite direction.
Contrarian betting: Systems based on betting against the public (contra squares) historically show modest positive results in certain markets. But market efficiency has largely arbitraged the most obvious contrarian edges, especially in NFL prime time.
Quality of data: Most publicly available betting percentage data is delayed, partial, or cherry-picked. Real-time dollar distribution from multiple major books would be the gold standard; most retail data falls well short.