Experience

According to Jeb during an interview, Experience Points will be added as a new aspect of gameplay to Minecraft. The development of Experience is being undertaken by Notch. Experience can be obtained by gathering Experience Orbs from defeated mobs and players. Gathering experience points will increase the player's levels by gradually filling a bar on the bottom of the screen until a new level is achieved when the bar is full. Each level up will reward the player with Skill Points, which Notch has said may be used for buying things such as increased jump height and increased maximum health.

Jeb_ stated on Twitter that experience orbs will not be obtainable unless you kill the creature yourself, meaning grinders will not give you experience orbs.

Jeb recently released a picture of the 1.8 GUI list, which shows the experience bar, among other things in there.

In 1.8 so far, there is no measurable benefit to gaining experience orbs and levels until 1.9.