It is pretty simple in that the better wins if all bets are correct. Parlays can include bets that involve point spreads, totals, futures, moneylines, and prop bets. If any of the bets lose, then you lose the money you initially put in. There is a potential for a higher reward because every bet that is made needs to be correct to win the payout.
A parlay is one big wager that involves two or more bets.