An Oran berry, when eaten by a Pokemon, heals the Pokemon by 10 HP. When it is a held item, it will restore 10 HP when the holder's HP goes below 1/3 of it's Max HP.
Poffin Uses: At 100% performance an Oran Berry will produce a level 10 Dry Poffin when cooking alone.
You level up your Pokemon by battling other Pokemon and defeating them, which makes you gain experience. When that Pokemon gains a certain amount of experience, it will grow a level. A Pokemon reaches it's peak of experience points when it reaches level 100. You can also level up your Pokemon by giving it a Rare Candy, which makes the Pokemon grow one level. Other items that help are the Lucky Egg, which lets you gain extra experience in battle, and the EXP. Share, which gives a portion of experience to another Pokemon that didn't participate in the battle.