Rest State

The Rest State modifier gives players a time-limited bonus to combat (PvE) experience. This frees up time for exploring other aspects of the game without penalty, such as tradeskilling and social activities, and helps players avoid level-grinding. Inns serve as a good place to hang up your sword or staff for the night because they confer special bonuses. Inns also act as home locations. Hearthstones provide a speedy way to travel back to an Inn when you need to take care of business in town, or when you're ready to check in and call it a night.

Rest State

• The Rest state modifier only affects experience earned from killing monsters. It does not affect experience earned from quest rewards or other experience rewards in the game (such as future PvP experience rewards).
• As players kill monsters and gain experience points, they become increasingly tired. Their rest level reflects this tiredness in two tiers: Rested = 200% experience and Normal = 100% experience.
• At level one, players will all start in the normal tier. When a player rests in a city or at an inn they will gain rest bonus at a very slow rate. Eight hours of rest will be needed for a player to gain one "bubble" of rest bonus. At any given time, players will be able to accumulate a maximum of 30 "bubbles" worth of rest bonus which translates into approximately 1.5 levels worth of rested play (before your character returns to normal rest state).
• Rested players receive bonus experience points for killing monsters. Normal players receive no bonus experience points for killing monsters.
• To become rested and start earning bonus experience from killing monsters again, players must rest for many hours.
• Thanks to the comforts of a warm bed and a hearty meal, players who rest or log out at an Inn will accumulate rest credit four times faster than players logged off outside of an Inn or City. Players who log out anywhere else in the world will earn rest credit four times slower.
• Note that while your character is resting at an Inn, you can play other characters on your account. The Rest State is per character only.
• Players have the ability to rest anywhere in the world. This helps players who forget or choose not to log off at an Inn. Resting at an Inn is still the preferred method, as rest credit will accumulate four times faster for players logged off inside of an Inn.
• There is no penalty for leaving an Inn early. You are constantly accumulating bonus experience while in an Inn or City. If you leave and come back, the only time lost is the time you were outside.
• Most players will be in the Normal experience tier most of the time. The Rested tier is primarily intended for players that take a day or more off from playing or are very casual with their play schedules. This has enabled us to greatly reduce the experience necessary for all players to level regardless of which rest state tier they play under.

Article from : http://www.worldofwarcraft.com/info/basics/resting.html