Magic requires runes of varying costs. Runes are stones that are created using runecrafting, purchased from other players or NPCs, receieved as drops from monsters. Magic can be considered expensive because of the costs of the runes if you choose not to make them yourself. In order to create runes, you must have completed the Rune Mysteries quest. This gives you access to the essence mine. Mining essence will give you rune or pure essence. Pure essence is available only to members. Rune essence is a weaker version of pure essence. In Old School RuneScape, everyone is a member so pure essence is available for everyone.
There are 3 spellbooks Magic users can cast from. They are Standard, 'Ancient Magicks', and Lunar. The Standard spellbook has a balance of combat and utility spells, while the Ancient Magicks spellbook has unique teleports but mostly combat spells. The Lunar spellbook has a higher level combat spell, but focuses mostly on utility. Ancient Magicks is unlocked after the completion of Desert Treasure , a Master level quest. The Lunar spellbook is also unlocked after a Master quest, Lunar Diplomacy.