Teasers are another form of accumulator bets.  Several bets combined for a single outcome. The difference between parlays and teasers is that with a teaser the bettor is given a set number of points to apply to each of his picks to enhance his chance of winning.  In exchange for the extra points, the payout odds are lower than for a parlay bet of the same number of picks.  Some books let you choose the number of points you get, with adjusted odds for each point total selected.  The more points you take, the lower the payout.

For example, a bettor wants to place a 3-team NFL teaser bet. His sportsbook offers 6 points on 3 pick teasers, and the teaser pays +180 (9/5) if all 3 picks win.

With the existing lines of:

Pittsburgh (-6.5)
Cleveland (+6.5)

Game Total (43.5)

and

San Diego (+12.5)
Houston (-12.5)

Our bettor places the following teaser bet:
Pittsburgh (+0.5)
Pitt/Cleve Over/Under - he takes Over at 37.5
and
San Diego (+18.5)

He has adjusted each pick by 6 points in his favor.  Based on his choices, if Pittsburgh wins, The combined score of Pitt/Cleve is 38 or more, and San Diego loses by 18 or less (or wins), our bettor will be paid +180, or $180 in profit on his $100 wager.

Rules on betting teasers can vary between sportsbooks, so make sure you fully understand the rules for the book where you are contemplating a teaser bet.

THE SPORTSBOOK'S ADVANTAGE

The sportsbook's advantage with teasers is that bettors rarely understand how much a point (or 6) is worth when they place a bet.  While a 3 pick parlay pays +600, a 3 pick teaser giving 6 points pays +180 (or worse).  To add to the complexity, the true value of the points is changes with the spread being offered.  The sportsbook understands this and many bettors do not.  Or worse, the bettor believes that his skill at applying the teaser points compensates for the book's overcharging for them through payout reduction. 

Let's look at a quick example of a two-team teaser giving 6 points and paying -110.  This is the same as a standard juice line, requiring the bettor to have a .5238 probability of winning in order to break even in the long term.  Winning the teaser requires winning both legs with a total probability of .5238.  Meaning:

(probability of winning leg1)(probability of winning leg2) = .5238

or the SQRT of .5238 = probability of winning either leg = .7237

You need to have a probability of 0.7237 on each leg to make it probable that your two-team teasers will have a .5238 probability of winning.  Question for the bettor is does the extra 6 points increase the win probability of each leg by .2?  If it does not, you are better off just straight betting the spread on each team.  (HINT:  if it usually increased the probability of winning a spread bet by .2 by adding 6 points to it, the sportsbook would not offer this option.)

What you need to discover is WHEN does adding 6 points increase the probability of winning by .2.  There are times when this occurs, and the only way to discover them is to know the value of a point.

Bottom line is that teasers are very complex betting situations mathematically, giving the sportsbook a greater advantage on most bettors.  While it is possible to find advantage plays using teasers, most bettors should steer clear of them until becoming thoroughly familiar with the odds involved. 

The problem is - they are one of the funnest ways to place a sports bet.