From Diablo Wiki
The Paragon Levels system is an end game feature. These levels are accessed once a character reaches the maximum level of 60. Additional experience gained from monster kills or kill spree bonuses counts towards the Paragon levels. There are 100 paragon levels, and each level up rewards a character with the same attribute bonuses as a normal level up (3 to the class main stat, 2 vitality, 1 to the other two stats), plus a passive bonus of 3% to Magic Find and Gold Find.
 Magic Find Changes
Other game changes were made with the Paragon system, chiefly to Magic Find and Gold Find. The maximum value for those properties was capped at 300% (not including Nephalem Valor stacks). Thus a character at Paragon Level 50 would have 150% passive bonus to their MF/GF, and would need only 150% more from equipment to max out their bonus.
Further, a character who reached the maximum Paragon level 100 has 300% Magic Find/Gold Find inherently, and up to 375% with 5 stacks of Nephalem Valor, thus would gain zero additional bonus from equipment that provides bonuses to those stats.
This system enables characters to cease wearing equipment for Magic Find gain as they level up, and focus entirely on survival enhancing stats. This design has proved controversial amongst fans, and has been objected to as a simplification of the end game item game.
 Official Details
- After a player reaches level 60, killing monsters and completing combat challenge streaks (Killing Blow, Massacre, etc) will begin to award experience towards Paragon levels
- There are 100 Paragon levels
- Every Paragon level will reward you with:
- Core stats such as Strength, Dexterity, Intelligence, and Vitality in amounts similar to what you’d gain from a normal level
- 3% Magic Find and 3% Gold Find
- In addition, a distinctive border will surround your character portrait in the in-game party frame to denote your Paragon progression, with a new frame earned after every ten levels. Your Paragon level will also be visible to other players wherever your normal level is shown.
- Nephalem Valor now grants +15% Bonus Experience per stack, as well as +15% to Magic Find and Gold Find
- Magic Find will now cap at 300% (Nephalem Valor bonuses will still stack beyond this cap)
- Magic Find is no longer averaged among all players in a multiplayer game
 Paragon Levels Experience
Players quickly datamined the patch for precise details about the experience gains required for Paragon Levels. Generally speaking, the first few levels are quite quick, but by Paragon Level 5 or 6 the gain is noticeably slower. Still, up to level 10 remains fairly quick, and then 11-20 is only a bit slower. The pace drops off considerably at that point, though up to level 50 is still not enormously grindy. Past 50 though, the number of kills per level increases exponentially, with progress very slow past that point, for most players.
Though the total experience needed is considerable, it was just two weeks before the first character reached Paragon level 100. The winner was a group-played Barbarian named Alkaizer, who dinged to the top on September 6, 2012.
Though one group of players was able to power up to it in just two weeks, most players will require months or years. Gaining Paragon Level 100 requires vastly more experience than leveling a character from 1 to 60 in the first place. More experience is required to gain Paragon Level 3 than to go from level 1-60 with a new character.
That's something of a misleading comparison though, since monster kills in Inferno are worth gigantically more experience than monster kills on lower difficulty levels. For instance, a nearly full clear of Act Three on Inferno is worth around 20 million experience, while it requires just 25,934,700 experience to take a brand new character from level 1 to 60.
Note that paragon level characters do not gain experience from quest rewards (though many players hope this will change). Paragon experience is awarded only from monster kills, and from bonus experience from destruction bonuses of objects or by scoring large monster kill streaks and mighty blow bonuses. (Though these other bonuses are fun to achieve, they grant a few hundred or thousand experience at most, which is considerably less experience than a character earns from killing a single monster in Inferno.)
 Experience Charts
A basic table showing the 1-100 level experience requirements. The figures show how much experience is required to gain the next Paragon level, not the cumulative total. Paragon level 100 requires slightly more XP than levels 1 through 17 combined.
A handy pie chart giving a proportional view of the total Paragon Level experience gain.
Several screenshots showing off aspect of the Paragon system.