Previously, once you reached level 60 in the game Diablo 3, you were done. There was no where else to go as far as progression of levels goes. Now there is something to strive for: Paragon levels. These levels will allow players to continue progressing in the game and achieving new stats and rewards, including an additional 3% magic find and gold find with each level. There are 100 paragon levels, and to advance a new level you will need to gain about as much XP as it takes to get from level 59 to 60. Along with the new levels and stats, your character portrait will get a different frame every 10 paragon levels so players can see your progression and achievements for themselves.
“In support of the Paragon levels, Nephalem Valor gives an extra 15% experience bonus per stack, which also applies to Paragon levels. Everything considered, Blizzard is comparing the time investment of the higher levels to reaching level 99 in Diablo II.”
For more information, check out the post on the Blizzard blog about the paragon system.