system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: AWD v 4WD

I have always assumed that awd drive is always on where 4wd you can turn on and off.  but recently have seen  advertisements for full time 4wd. and have had people tell me that their 4wd is always on.  Doesn't that make it AWD?
gpt
Typically a true "4WD" vehicle has a transfer case as opposed to a differential.  The fore and aft shafts are locked so the front axles and rear axles move at the same speed.  An AWD car usually has a differential of some sort.

A transfer case or fully locking differential is almost a requirement in a true off-road vehicle.  With an open differential you'll find yourself motionless if one pair of wheels is off the ground or otherwise spinning. 

The downside is that a transfer case is bad news on dry pavement.  If the front and rear are locked together, and there's a lot of grip because the pavement is dry, any time you go around a corner you're wearing the transfer case and putting unnecessary wear on lots of other stuff (tires included).