PUG is an abbreviation for Pick Up Group. Refers to a party assembled from the random pool of available players in a town, as opposed to a coordinated Guild excursion or friends setting up an adventure together.
PUGs are easy to find and assemble, but will usually be of inferior quality than organized groups, the main reason being lack of communication and familiarity with each other's style. PUGs are most convenient for quickly doing a quest or mission and are least suitable for areas of the game that require a great amount of synchronization such as Tomb of the Primeval Kings or the Fissure of Woe.