Guildleves are Final Fantasy XIV's quest system. There are various types of leves, all offering different challenges and rewards for the player. There will be some guildleves where a solo player can complete them within half an hour of play, which is ideal for the casual gamer who does not have a lot of time available.
If you don't want to do them alone then you can take a group with you and everyone will get the reward. Guildleves may ask you to do tasks such as hunting monsters, raids, harvesting or give you access to restricted areas.
Upon requesting a leve from the guild, the guildmaster will assess your skills and decide which leves are available for you. This basically means that you won't be able to access the whole collection with a starter character; think how certain quests in Final Fantasy XI had a level requirement.
At GamesCom in Germany in August 2009 we saw the guildleve system in action. People were allowed some time to demo the game and complete a guildleve which requires players to kill 6 Dodos and 6 Puks. Upon completion an aetheryte crystal opens up and the participants receive a gil reward.
It has also been revealed that some guildleves will allow players to fight tough monsters like the HNMs in Final Fantasy XI. These will be instanced, meaning no one can interfere with your battle and no camping for the spawns. There will still be some NMs in the game that randomly spawn however.