Full time 4wd means the vehicle is constantly providing power to all four wheels. Usually power is shifted between the front and rear axles as needed by the transfer case and center diff. This provides the best traction in both dry and slippery driving conditions. This requires no action from the driver to activate. Some Toyota and Nissan vehicles are full time perminant 4wd.
Part time 4wd or perminant part time 4wd means the vehicle has the drive train to allow all four wheels to recieve power from the engine at once. The difference is that part time 4wd allows the driver to choose when 4wd is activated. This can be done by manually locking the front hubs or by a lever available to the driver in the cabin.
4wd vehicles have 3 settings. 2H is a high ratio geared 2wd mode allowing you to drive with ease on the road and highway like a normal car. 4H is a high range 4wd mode allowing the vehicle to travel on gravel and slippery surfaces at relatively high speed. 4L this puts the vehicle into low range and is best for 4wd obstacle climbing or water crossings. 2H gears down the engine to give it a major increase in torque at lower rpm. This in turn means you get more traction and a much more stable drive over an obstacle.
The gearing options found in 4WD vehicles help a vehicle tackle many of the unique situations encountered while driving off-road. Following are a variety of conditions in which you would want to use 4WD to avoid sliding or spinning. 4H allows you to drive full speed, if necessary. The high range ratios in 4WD mode are the same as the gear ratios in 2WD.When "4-low" is selected the wheels create substantially more torque (for example on a Grand Cherokee its 2.72 times more) than in "4-high" - at the same time the vehicle moves at substantially slower speeds (2.72 times slower on a Jeep Grand Cherokee.
The 4wd low range does not create more traction - it creates more torque at slower speeds and that can be detrimental when the ground is slippery. Slipping tires are more likely in "low" than in "high"! On snowy, icy roads "low" would be a poor choice - some really deep snow, however, puts up so much resistance that "low" is needed to push forward.
If low range 4wd is not enough to get your vehicle out of a bog, try Tough Toys 4wd accessories for some great recovery gear to help you get unstuck!