Powerleveling is where a player hunts constantly to try and gain experience points. They often leave loot in the bodies of the creatures they have slain.
Knights and Paladins: Instead of saving and looting items, they simply hunt all day, regardless of the consequences. Often hunting places include Frost Dragons, Dragon Lords, Orc Fort, Wyrms, Bog Raiders and Vengoth because of their high yield of experience. This leads to High Level Character, but, unfortuantely, low skills over all. It is best to train and Level in a balanced manner, or train much on low level, and start hunting with skills 80 or more.
For mages this is entirely different: They use wands and spells to hunt with, which absorbs mana, they either have to take a lot of Mana Potions, also things like soft boots and life rings are useful. Before 2010 Winter Update sorcerer used to be the fastest leveling up vocation. But then cool down has been introduced and now spells like Energy Wave and Rage of the Skies can only be used after waiting certain time.
Sometimes when Powerleveling a lower level character would usually use GFB runes or Avalanche runes, which can be quite expensive. Some very high level mages powerlevel on Dragon Lords using Avalanche, or Deep Banuta using Rage of the Skies and Thunderstorm. Knight usually hunt with Druid to reduce healing costs. After 8.7 update knight can hunt solo without wasting a lot health potion because of high amount hp healing by Wound Cleansing and Recovery, and attacking spells like Berserk, Front Sweep, Brutal Strike