Gambling terms?

Somebody school me

I understand the over and under concept but what does spread mean? How does that work? As far as I know, the underdog is given a certain number of points by the other team. This number of points must be met after the final score? I don't get it?  Can I get an example?

lets say the chargers are playin the colts.  the colts are favored by say 11.5.  so they automatically give the chargers 11.5 points to start.  so if the score is 20-20 the score with spread is 31.5 chargers-20 colts.  so basically the colts have to win by at least 11.5 to cover the spread.

So in your example, the chargers win b/c the final score is 20-20 and the colts did not cover the spread?  

so in "spread" terms the score is 31.5 (chargers) and 20 (colts)?  so therefore the chargers win the bet?

Also in Vegas and I am pretty sure online gambling sites as well, ties win. So if say the colts are favored by 2 points and beat the patriots by 2 points. If you chose either the colts or patriots , you would win.

ties lose my friend, maybe if you are lucky 'local's will let ties win..Ties with the spread usually = no blood (not a win/not a loss)