In football betting, a "line" is provided by which the favorite team needs to win, in order for bettors to win that game.
Sometimes the line is x.5 points. If it's impossible to gain half-points in football, why have a half-point line?
This is done to ensure that there is not a push when the game is over. A push is when no one wins, and the sportsbook gives everyone their wager back.
Here is an example:
For this week's Pittsburgh at Baltimore game, one sportsbook lists the point spread as 3, with Pittsburgh favored to win. Let's say you decide to bet on Pittsburgh. If Pittsburgh wins by 4 points, you win. If Pittsburgh wins by 2 points, you lose. If Pittsburgh wins by 3, you get your money back, along with everyone who bet on Baltimore.
Another sportsbook lists the point spread as 2.5. If you bet on Pittsburgh, and they win by 3, you win, and if they win by 2, you lose. There is no possibility of a push in this case.
Further reading: Maddux: How NFL Lines Work