Pools, cards, and parlays - A parlay is just a combination bet. Instead of making five separate bets on five different games, the bettor places a single parlay bet, hoping he or she can predict the outcome of all five games. If any one of the games is a loser, the entire parlay is a loser. The payout is better for parlays, because the odds of successfully picking multiple winners are much tougher to beat [ref]. Most people are familiar with parlays through office betting pools or football cards. If you've ever paid $5 to select your winners from a list of that week's football games (sometimes using the spread, sometimes using the straight scores) in hopes of winning the weekly prize, then you have made an informal parlay bet. Office pools don't usually involve actual bookies beyond the person who organizes the pool, and no one takes juice from the pool. All the money is distributed to the winner or winners.
In general, the betting public tends to gravitate towards favorites when betting the games regardless of the actual pointspread. This is especially true with high-profile teams such as Dallas and Green Bay in the NFL and Golden State and Cleveland in the NBA. The sportsbooks are well aware of this phenomenon and often times they will adjust the betting spreads accordingly. This, in turn, actually adds some value to the underdog when you consider that a pointspread is nothing more than a handicapping tool that is designed to even out the match.