whats a PUG?

ive been playing for 21 months, and nobody will tell me what a PUG is... ppl rarely say it online, and i see it alot here...is it like a npc?or what

PUG stands for pick up group

Yeah its a bunch of people that you dont really meet with before hand for a mission or something like that.

Guildies arent a PuG

If you join a group of people you don't know, it's a pug. If you have a group of guildies/allies and take a random person to fill the group out, they are a pup (pick up person).

note that in PvP (specially GvG) a PUG usually stands for a guild that plays with members/guests from various other guilds.
Much like a "smurf" guild but without any actual connection to one specific guild.

in general, PUG means Pick Up Group, like lucifair already said

PUG: A group that will either garner your appreciation for guildies/heroes, or cause you to desperately seek out a guild/acquire heroes.

What? You mean it's not a breed of dog?

PUG - noun - The generally derided, often maligned, rarely cooperative, loosely coordinated beast with 8 bodies and no brains. See also rage quit; leecher; etc.

That being said, some of the best ppl I met in game were from that rare breed of pug - the organized one that gets the job done.