There's a live straddle and a non-live straddle.
Non-live straddle - basically means UTG min raised blind, if no one raises preflop, UTG doesn't get a chance to make a raise.
Live straddle - UTG min raised blind, but this bet is treated as a 3rd blind. In a 1/2 NL game, UTG live straddles to $4. If there is no raise preflop (say everyone calls, BB calls as well), now UTG does have the option of raising if they choose.
Quite a few casinos will allow a live straddle UTG, but subsequent "restraddles" will not be live.
There's variations of straddles, such as the "downtown" straddle that Binion's and The Golden Nugget use in Vegas. It's a live straddle placed on the button, button acts last preflop (ie, after the blinds). There's also no limit on the size of this straddle ie, in a 1/2 game you could straddle $5 (a common amount) or more. This is similar to a Mississippi straddle.