In its broadest definition, a bet is a catch-all term for every wager made in poker. In a more narrow meaning, a bet is an initial wager, after which other players have to decide whether they want to fold, call, or raise. A 3-bet, 4-bet, or 5-bet is also a different type of preflop bet.
A bet can be made for value or as a bluff, both preflop and post-flop.
For example, in a Texas Hold’em game, your opponent raised preflop and made a continuation bet after the flop. In this case, both preflop raise and c-bet on the flop would be considered a bet from your opponent, even though the initial preflop bet is more commonly referred to as a raise.