Quests are plot based series of tasks that give a player the chance to witness character development within the storyline. Players recieve rewards for completing quests, some of which are unique and cannot be aquired otherwise, such as Ava's Accumulator.
The quest journal is a tab on the player interface section of the game, which displays the quests available to both free-to-play and member players. This is no longer nessecary, considering that membership fee is required for OSR access, which leaves one to wonder why it has yet to be programmed out. Some speculate Jagex is developing a free-to-play version and worlds. The achievement diary can also be accessed through the quest journal tab.