What is the difference between 235-70r-16xl and 235-70-16?

235 -70-16 is just a size. It is not a valid description of a tire. The r in your other description could be R for radial or D for Bias/belted tires.

So the 235/70R16XL Is that size radial tire rated for Extra Load, Extra Load is the XL at the end. You need to be careful though, there is a rated weight printed on the side wall that is the max your tire can Carey safely