A parlay, or accumulator, is a bet where multiple selections are combined and a return is given only if all selections wins.
The obvious advantage with parlays is that the possible payoff is much higher than when all selections are played as singles. The main disadvantage is that the risk of losing increases with every added selection. Only one selection needs to go wrong for the parlay to fail.
If a selection in the parlay is voided or pushed (in Asian handicaps) that selection is removed from the parlay and the number of selections decreases, e.g. a treble is reduced to a double in case of one push.
Selections that are not mutually independent are usually not allowed to combine into a parlay. However, some bookies allow a parlay from specific selections in the same match, for example a combined bet on over X goals and over Y corners in a football match can be allowed.
To gain something if a single bad selection is made some punters like to place combinations of accumulators from a couple of selections. For example, four selections can be combined in four trebles or six doubles.
Most professional tipsters do not play parlays since the probability of a win is small. This results in long runs without wins and a need to afford these long runs. Moreover, when playing single bets you can always compare different bookies and place your bets on the bookies with the highest prices. It is hard to find a number of matches with good value from the same bookmaker. Therefore, the expected value of the long run return is usually higher if single bets are played instead of risky accumulators.
Sometimes media (often sponsored by the gambling industry) reports of crazy parlays that resulted in huge wins with small initial wagers. What they do not report on is all the losing crazy parlays that many tipsters play. Once again, the probability of winning with a parlay decreases for every added selection. With that said, there are punters that claim that they have found winning strategies with parlay bets.
Parlay betting is one of plenty of words/phrases/abbreviations in sports betting I have written about. See this dictionary for all the terms I have tried to explain.