They've added one more wild card position in each league (so, now 10 teams as opposed to 8 in the post season). This creates a new one-game, wild-card round in the AL and NL between the teams with the best records who are not division winners.
From wiki:
Also, with the adoption of the expanded playoff format in 2012, the five-game Division Series will begin with two home games for lower seeds, followed by up to three home games for higher seeds. This one-year change will eliminate a travel day prior to a decisive Game 5 of a Division Series and was necessary because the 2012 regular-season schedule was announced before the agreement on the new postseason was reached. For 2013, the Division Series will return to the 2-2-1 format used in previous years.
From wiki:
Also, with the adoption of the expanded playoff format in 2012, the five-game Division Series will begin with two home games for lower seeds, followed by up to three home games for higher seeds. This one-year change will eliminate a travel day prior to a decisive Game 5 of a Division Series and was necessary because the 2012 regular-season schedule was announced before the agreement on the new postseason was reached. For 2013, the Division Series will return to the 2-2-1 format used in previous years.