The term big rig is a euphemism for a tractor trailer
, at least in the United States. The strict definition is somewhat amorphous and probably varies depending on who you ask.
Is a U-Haul rental truck a big rig? No, it's a straight truck (or box body). The term straight has nothing to do with the sexual proclivities of the truck or the driver. It doesn't have a 5th wheel, which is the joining point between a tractor and trailer. Absent the 5th wheel, one can back a straight truck without considering the extra joint, making it a much easier operation.
Are all tractor trailers called big rigs? Yes, but again it depends on the source. A 53' dry van behind a 240" wheelbase Peterbilt is a big rig by any definition. A 28' marine chassis with a 20' container behind a daycab is much smaller, but I submit it is a big rig still because it consists of both a tractor and a trailer.
There are many combinations of axles, wheelbase, sleeper and non-sleeper configurations, trailer types, etc. The logic of defining a big rig as a tractor trailer combination is simply a means to draw a line between certain types of vehicle.
Is a tractor going down the road without a trailer a big rig? No, that's a bobtail. It currently has no trailer attached, so by definition wouldn't be a big rig until it is hitched to a trailer.
So, are all truck drivers big rig drivers? No, not all truck drivers are big rig drivers. Most straight truck drivers (here we go with the sexual innuendo again) drive vehicles with a gross vehicle weight rating (GVWR) of 26,000 pounds or less, which alleviates them from compliance with certain federal regulations. Those guys are simply truck drivers. Usually it's the guys with the tractor trailers who go over the 26,000 pound threshold and enter the world of big riggers.
Who is this guy I hear about on the CB radio, Billy Bigrigger? Billy Bigrigger is to trucking what Barry Bonds is to major league baseball. He's part of the fraternity but not always a welcome part.
Billy has seen it all, done it all, and knows it all. He's the one stuck in 1977, living out Burt Reynolds' role in Smokey and the Bandit. He has the dinner plate belt buckle, the chain drive wallet, half a chicken dangling from his cowboy hat. He's also the one who takes a half hour to back his rig up to the loading dock door instead of the normal 3 minutes used by an actual truck driver. He can talk smack on the CB from dusk until dawn but he can't walk the walk.
So, how is Billy different than any other trucker? Billy Bigrigger wears cowboy boots.
What's the difference between a cowboy and ol' Billy Bigrigger? A cowboy has the bull crap on the outside of his cowboy boots