What are the quests called in the elder scrolls games that you can play endlessly in the guilds the series gives you

They exist only in Skyrim and are called 'Radiant' quests. Each time you ask for one, it will be randomly generated for you. They can include,

Killing a dragon, giant or bandits for the Jarl, given by innkeepers,
Hunting a wild animal for the Companions,
Stealing items from a house for the Thieves Guild.