Lots of moves have higher and lower speed priorities, which override a pokemon's usualy speed stat. Extremespeed and Quick Attack are both the same priority though (+1) so they will act like normal moves and whichever pokemon is fastest goes first.
The serebii link narwhals posted is ok but there is also Pursuit which is +7 only when the opponent is switching out, and switching out itself (+6) which comes after Pursuit but before everything else.
UPDATE: ExtremeSpeed is +2 priority in Black and White.