What is the deference between OFF- and DEF+? Are they not the same?

Blades generally range from OFF+, OFF, OFF-, ALL+, ALL, ALL-, DEF+, DEF.

So you can see that there are 3 levels between OFF- and DEF+.
I am far from an expert on blades, but basically DEF+ is softer and slower than the OFF-.
If you are playing a chopping, defensive game, then you want to look for a blade that is slower like a ALL- or DEF+ blade.

Thanks.

Is it possible one side is slower than the other side? In the this case, the blade is offensive but little slower on the back hand side.

I think it is the same for both side. To make one side slower, it depends on what type of player you are. If you are the offensive type a blade with OFF+ or OFF will be better. You may put an "offensive" rubber on one side and a "defensive" rubber on the other.