PUG is just a pick up group and has nothing to do with RNG...
any group that has 1 or more players that are unknown is a pug...
its like a pick up game of soccer or basket ball... you show up, you pick a team, you play.
for WoW PUG's you enter LFG queue, you join a group you instance.
PUGs are usually not effecient as you rarely get the group cohesion you get with guild groups..
however, sometimes pugs are just good players who cant get a group with their guild at that time.
there are several levels of a PUG
1. Lucky pugs: a group of random players join and make a party for an isntance where they are all experienced players and the instance goes by without a hitch; these aren't as rare as you think. but they are the rarest of them all.
2. the completley ignorant PUG: a group of random players who have no idea what they are doing but it doesnt matter because nobody knows that the other doesnt know what they are doing.. so everybody is ignorantly playing how they usually play; these groups aren't as rare as you think (if you are a good player see #3). This group tends to fall apart quickly as somebody usually has to go, somebody dc's or the group decides the instance is too hard.
3. the UNlucky PUG: an experience player gets stuck in group #2. this is a common type of pug.
4. the not-so-much-fun PUG: an experienced player joins your rag-tag group of misfits and talks like he is all high and mighty and knows everything and makes the whole experience miserable (he thinks he is in group #3). this is the most common type of pug.
5. The 25% pug group, a group that consists of 3, 20 , 6 players in the same guild. they always blame you for mistakes. they always insist on 3 CC classes and wont invite you because you are a shaman. or they wont invite you because they need an off healer and you ARENT a shaman.
5b. the 99% dont need you but take you PUG: you fill in for a guild member in a group of 4, 9 or 24. they dont really need you but they figure they have an empty spot. you play really really well, and they ask you to join their guild. BINGO GROUP!
those are the pugs i encounter.. lol.