They first fought, of course, in 2004, and it was an electric bout. Pacquiao dropped Marquez three times in the opening round, but Marquez came back to put on a boxing show for the ages, earning a draw (scores of 113-113, 115-110 Pacquiao, 115-110 Marquez).
Since then, Pacquiao has gone 7-1 (4 KO, loss to Erik Morales) and become one of boxing's biggest drawing cards. Marquez, on the other hand, has gone 6-1 (2 KO, loss to Chris John), but hasn't been able to make his name thanks to some questionable (to say the least) business decisions by himself and Nacho Beristain. Marquez gained his long overdue star-making win in March over Marco Antonio Barrera, whom Pacquiao retired in October.
The bout will be fought for Marquez's WBC super featherweight title, so there won't be a catchweight or anything. If Pacquiao is focused for the fight, then there's no reason he should be having so much trouble making 130 pounds. He'll need to be strong for Marquez, who has gone against the grain and gotten more aggressive as he's aged.
This is a hell of an interesting fight, and should be a good one. At 34, Marquez probably doesn't have a ton of time left to step up to that final level of superstar status, and this is the fight he has to win to do it. For the 28-year old Pacquiao, this is a chance to basically conquer all of the Mexican fighters he's fought. Marquez, as Oscar de la Hoya put it, is "the last Mexican standing."
As noted by Matt, it will also be for the vacant RING 130-pound title, as Pacquiao and Marquez are rightly ranked 1-2 by the publication. We'll have a linear champion again, though if Pacquiao wins and moves to 135, we're not likely to have one for long.