I've seen this term used in various questions, but have no idea what it means, so could someone please define or explain it for me please?
A straddle refers to an extra blind bet made preflop. Typically, this is made by the first player after the big blind (e.g. the Under-the-gun player), however in some variants that is not the case (some allow the button to straddle; others allow straddles from any position on the table). In the end, it depends on the rules of the casino. In almost all situations, the player who makes the straddle is given the right to act last preflop. Postflop order of play is unaffected (and proceeds the same as it would if there were no straddle).
For more information on the types of straddles and when they might be useful, see http://www.parttimepoker.com/understanding-the-straddle-in-texas-holdem-poker-facts-thoughts-and-strategies
A straddle is when someone takes the option of the big blind by putting in a bigger blind. For example, if we had $1-$2 blinds, the first person after the big blind could "straddle" by putting in a $4 blind. They not only raise the blinds but can take the last to act preflop position, if the straddle is "live".
A straddle is an extra blind. It is typically double the previous big blind (some games may allow other amounts, especially at no-limit), and becomes the new big blind amount. Generally this is done in UTG position, and it is "live", in that if there has been no raise when the action gets back to the straddler, he has the option to raise. Some casinos allow a straddle in other positions. Some allow multiple re-straddles, so that a 1-2 blind game, for example, might have a $4, $8, and $16 restraddles making it $16 to go.
The term "kill" is often used for a straddle that not only raises the big blind, but raises the betting limits for the whole hand. These are often posted out-of-position and made mandatory on certain occurrences like scooping in a high-low game.
Finally, there is the rare, but fun, "sleeper" straddle which is a straddle always out of position with the following rule: if all players between UTG and the sleeper fold, then the sleeper's straddle is "on", and acts just as if it has been an in-turn straddle. If, however, someone before the sleeper calls or raises, it is "off" and he can take whatever action in his turn he wishes.