From the RuneScape Wiki, the wiki for all things RuneScape
Rune running used to be when a player (the "runner"), brought rune essence or pure essence to the altar, which he/she traded to the crafter in exchange for noted essence, runes, coins, or a combination of the three. With the release of skill assisting, rune running has evolved. It is now the act of a runner requesting assistance from the crafter using the assist system. The assisters let the runners borrow their high Runecrafting levels to craft multiple (or on the case of some, single), runes from one essence. In exchange, the assisters get all of the Runecrafting experience, while the runners get many more runes than they would have gotten if they crafted the runes themselves, however they do not earn the Runecrafting experience. Most runners have low Runecrafting levels, and most assisters have high Runecrafting levels. Rune running can be beneficial to the runner, by helping them earn money, and it can also benefit the crafter, by saving them running to the bank and back to the altar again to craft runes. For a detailed guide, please see the following guides:
The most common kinds of rune running are nature, law, and air. Less common rune running are chaos and mind rune running. Water rune running is the newest rune running, due to an update which allowed the price of water runes to rise above the price of air runes, making them more profitable.
Multiple rune profit table.
Official rune running worlds