How Las Vegas Sets Sports Betting Odds:
...in sports like Baseball, Football, Hockey, Basketball, Soccer etc. a line is made by a casino line maker based on his or her analysis of the matchup and the anticipated outcome. The casino is essentially a "bookie." After the "opening line" is set, then the line fluctuates based on where the public's money is going. The casino's goal is move the line up or down so that the money is balanced on either side of the proposition and the casino has minimum exposure and makes its money off the vig (the amount it charges to make the bet). Most of the time, this works just fine for the casino, but sometimes line makers are wrong and the casino ends up with unbalanced money and has exposure to the proposition.
http://www.quora.com/Sports-Betting/How-does-sports-betting-odds-work