Board Thread:Frustrations and venting/@comment-17210794-20130921023430/@comment-11490363-20130925152618

It does not reduce crit chance as far as we can tell, but it does reduce damage done, all other things being equal.

That said, the amount of damage reduction is not huge. going from 10K defense to 20K defense gives you maybe an additional 10% reduction, so it is usually better to just max Attack and HP instead of defense.

There are some detailed calculations in an older thread, look for 'defense'