Basically, a Guildleve is the ‘card’ presented upon the acceptance of a Levequest. These are the bread and butter of FFXIV, the basic treadmill quests used to obtain Gil and items.
From the Adventurer’s Guild in Limsa Lominsa you can accept various kinds of Leve to undertake. Generally these will either be to eradicate certain enemies, collection of items dropped from monsters or harvesting of certain materials.
Once you have a Leve and go to the point of activation you will be asked to select a difficulty based on how many people are with you (solo or party). Higher difficulties bring in higher rewards. Each Leve has a time limit and this is shown to you before accepting, along with your reward and potential bonuses.
When you are accepting new Leves you may trade in completed Leves to receive better rewards. Upon completion of a Leve an Aetherial Gate will spawn next to you. This can be used to restore HP/MP and teleport back to the acceptance point of the Leve (indicated when you accepted the Guildleve – usually Camp Bearded Rock early on for example).Leves have rank and discipline restrictions, pay attention to these when you accept. The targets you must kill for Leves cannot be attacked by people not undertaking the Leve.