A straddle is completely optional and risky (for lack of a better word strategy).
As the definition Stormswa posted it is a blind bet wich is double the big blind in limit poker and a minimum of double the BB anywhere up to your whole stack in no limit. The next person to act may also re-straddle, that is re-raise the initial straddle (minimum of double the bet), also blind. The rules may be different in the US, but in the Sydney casino the UTG straddler does not get to bet last preflop (as the definition suggests) as the straddle is his bet, and if it is not raised he cannot bet again.