What is AWD? Can all four wheels not be powered at the same time?
Is there any real difference if in snow?

Thank you.

Robert, it would serve your purposes far better to do some extensive online research regarding 4WD, and hokw it works. What you are getting here is hit-and-miss info for a situation you have either not explained clearly,or we have not understood. To be asking at this point what AWD is tells me you need a lot more info than most of us have time to give, or explain. I am not being negative about your posts, but am trying to give you the best advice to help you better understand what you need.

If anyone is upset by my comment, I apologize in advance.

AWD and 4WD have become misleading marketing terms, more than anything.
AWD typically implies a system that is constantly active while 4WD requires a manual engagement and not suitable for dry conditions.

AWD itself can be divided into two categories - part-time and full-time.
Part-time will only engage the second axle when needed, while only one axle will actually drive the car most of the time.
This would be a typical implementation and most consumers would not be any wiser. RX slides right into that category.

Full-time AWD systems are a bit more complex. They send power to all 4 wheels all the time and have to compensate for all 4 rotating at different speeds, depending on steering input and other factors.
That's why having a center differential is integral. Lexus GX and LX both have full-time AWD, which marketing calls full-time 4WD. Toyota 4Runner Limited also happens to have the same center differential, while all other trims do not.
All Subaru vehicles used to have center differential as well, but their latest implementations appear to be a hybrid of both part time and full time... and a rather good one at that.
Full-time AWD is a dying breed, with EPA waiting to put the last nail in its coffin in the name of fuel efficiency. Most modern vehicles with AWD are switching over to part-time AWD, some with very trickery engagement.

Rather than a center differential, 4WD vehicles have a transfer case. In my understanding, all it does is rotate front and rear axles at the same speed when 4WD is engaged (I'm not 100% clear on its internals). Driver has to pull a lever or press a button to disconnect second axle under normal driving conditions as it will cause premature wear and high fuel consumption. It also has a Low 4WD gear to send lots of torque to the wheels, capable only of crawling speed. This is good for extreme off-road situations and to pull other vehicles out of a ditch.

The reason why Lexus RX and its younger sibling Toyota Rav4 may be marketed as 4WD is because the rear axle is connected to the engine via a clutch coupling and has no center differential to cope with different axle rotating speeds.
The engagement is completely controlled by electronics and no slip needs to occur for it to work. It will proactively engage when starting from a stand-still and will disconnect when turning. Once at speed, only one axle drives the vehicle.
In off-road conditions, it will overheat and the vehicle will become FWD-only to prevent any permanent damage.

Subaru and Audi were the early adopters of full-time AWD and have built their architecture around it. For most vehicles, AWD is an after-thought.

That's my 2c. Hope that helps.

Understood. I know what AWD is and the 4WD in our Expedition and Jeep Cherokee.
Just trying to learn what the practical differences are and if Lexus has their own design/characteristics.
Have never locked-in the RX350 AWD and wonder if there is ever a need to lock it in.