Hi, Affluenza
It's a good question actually...
If you raise from the SB pre-flop, and get called from the BB, then the BB is last to act post flop.
It all starts with the dealer or "button".
In modern play the first player to act makes a bet "blind" he must bet before he sees his cards.
The next player technically min-raises that bet -- also "blind". He must make the bet in order to see cards.
These are called the small and big blinds.
Although forced, these bets are live -- in other words, even if called, the big blind can re-raise. Other than this one time, a player who calls the raiser stops the action.
Post flop, the Button is always last to act.
This gives him the advantage of watching every other players actions before making his own decision.
If the button folds, the CO, then MP etc become last to act.
This is why you can open wider from the BTN.
If you open raise, the SB and BB must have pretty good hands to come along, because they will have to act first.
So, to sum up:
You are "In position" if the other player must act first.
You are "Out of position" if you must act first.
In multi-way pots, a person can be both in position and out of position depending on which other player you compare him to.
An example:
MP bets preflop.
BTN calls
BB calls
BB must play first post-flop.
He is out of position.
MP acts next.
He is in position compared to BB, but out-of-position compared to BTN
BTN is in position compared to both.
I hope that helped...
--VS