Where does the power go ?
If you are a purist (read anorak) of 4X4 then these things matter.
Things like the RAV are permanent 4 wheel drive, and you have no control over where the power goes.
Not sure how much detail to go into here before it becomes a mechanics lesson so I'll do my best to keep it short(ish).
Road vehicles have a differential gearing that allows the wheel on one side of an axle to turn at a different speed to the other. This is needed because as you go round a corner the wheel on the inside of the curve travels less distance than the wheel on the outside. If you had a solid axle the result would be massive trye wear as they fought against each other. Effectively one or both would be 'skidding' for the duration of the curve.
If you look at the underneath of a disco for example you will see a bulge, about the size of a small football in the back axle, and another in the front axle. These are the 'diffs'. What you don't see is that there is also one in the middle to allow the front wheels and back wheels to be driven at different speeds to, or to control what percentage of power goes to what end of the car. Most 4X4s do not apply equal power to front and back, there will be something more akin to a 60 - 40 split but this is a design feature of the manufacturers choosing.
What this all means is that if you lifted up one wheel of a normal car and put it in gear the wheel off the ground would turn, but the one on the ground wouldn't. The diff puts the power to the path of least resistance - in this case the wheel off the ground. Please do NOT try this - take my word for it.
If you take a 4X4 for MOT when they do the brake test they need to take this into account because turning the wheels of just one axle will, via the connecting axle and diff try to turn the other axle resulting in very expensive damage (that's why there is a warning sticker to this end on mostg 4X4s).
So your 'normal' 4 wheel drive doesn't allow you to choose where the power goes. It is either by 'dumb' mechanical diffs or more recently electronics. Either way it isn't you that chooses.
Compared to . . .
Vehicles designed for more off road use will have combinations of :-
High / low ratio boxes.
2 or 4 wheel drive (may be selected electronically on the run with a dash switch - like my now long gone Ssanyong Musso) - or by some sort of gear lever (normally at low speeds only), or even by having 'freewheeling' front wheel hubs that you need to get out and select / de-select.
Centre diff lock. Allows you to lock the front axle to the back axle. This means that if, for example the front wheels have no grip the diff is prevented from putting all the drive there and made to give some to the rear wheels, thus hopefully getting you unstuck.
Cross axle diff lock does the same but on one axle, so if the left wheel is spinning but the right has grip selecting cross axle diff lock will force both wheels to turn, once more heopfully getting you unstuck.
Add the two together, cross and inter and now any one wheel having grip will provide movement.
HOWEVER - and note this well. If you lock either or both the diffs on normal driving conditions you will cause serious and very expensive damage to your vehcile as the differing road speeds present at each wheel will set up stresses in the drive train. It may also seriously affect your steering and overl handling.
Diff locks are only to be used in adverse conditions and unlocked as soon as possible.
Hmmmmm Sorry that went on a bit but hopefully explains it.
I used to drive an 8 (double back axles and double front axles) wheel tipper on quarries. I could lock inter axle (left to right) or inter axle (both backs) or both. Selecting them both always got me out of trouble but at the cost of steering. Nearly put me over a cliff face once when I forgot to unlock them and tried to steer.