Spread-limit (usually shortened to just Spread) is a betting structure in which a player may bet or raise any amount within a specific range. For this reason, a player generally cannot overbet a pot in a Spread game.
Spread-limit games are described by stating the range of the spread, in the form "small - big", for example "2-6 spread" or "3-10 spread". There is only rarely a maximum buy-in for a spread-game, but as with all forms of poker, there is a minimum buy-in. The spread's range does not change for later betting rounds; in a 2-6 spread game, a player may bet or raise between 2 or 6 dollars in any round.
The minimum bet (the low end of the range) in a spread game is usually the size of the big blind; if it is any amount other than that, the game will explicitly state what the blind amounts are. Just as in no-limit games, raises which come after an initial bet in a betting round can be no smaller than the previous bet or raise, so as each bet or raise is made during a betting round, that bet or raise becomes the new minimum for that round. Obviously, once one player has bet or raised the maximum of the range, the remainder of the betting round plays as though it were fixed-limit: players can only bet or raise by that amount. The minimum bet size is then "reset" at the start of the next betting round.
Spread limit is becoming an endangered species of betting structure. At one time, it was seen as a way of "splitting the difference" between the safety of fixed-limit and the excitement and strategy of no-limit. While the player has some sort of control in indicating strength or weakness through bet size, and can control the odds they lay other players to some degree, they generally cannot "lose everything in one hand", since an opponent cannot suddenly bet $1000 into a $100 pot.
However, with the advent of the poker boom, the huge influx of new players want to play no-limit and find the excitement worth the possibility of losing everything in one hand. Accordingly, since the late 90's, more and more former spread-limit games have been changing to no-limit instead. Very few casinos have true spread-limit anymore.