The first three numbers is the width in mm of the tire, therefore the 245 is going to be 20 mm wider than the 225. The next two numbers is the height of the tire, expressed as a ratio to the first three numbers. Meaning, the P225/75R16 has a height that is 75% of 225 mm (168.75 mm); the P245/70R16 has a height that is 70% of 245 mm (171.5). So contrary to Doug's post, the P245/70R16 is going to be wider and taller than the P225/75R16. Now - to answer your question - yes, P245/70R16 tires will fit the stock Grand Cherokee Rims. I have a 2000 GC and they fit just peachy. They are also listed as alternate tires by tirerack.com. The handling doesn't change isn't too noticeable, but it is slightly wider, putting more rubber to the road and theoretically improving handling, traction, etc. I haven't noticed any change in fuel mileage, negative or positive.
