Defense reduces the number of times you're hit.
Resistance reduces the amount of damage done when you actually get hit.
0% Defense
50% Resist mean, if something hits you for 200 points of damage, you see 100 points. EVERY TIME.
With Defemse, you either get hit or you don't (as in zero damage).
There's a formula for determining probable damage over time.
This old post has Arcanaville explaining it in a concise, complete manner (so I don't have to...lazylazylazy....)
And actual damage mitigation over time is
Zero Resist, and Zero Defense x total POSSIBLE hits modified by (Successful Hits minus Resistance) minus failed To Hits.
Plug in numbers from here: https://archive.paragonwiki.com/wiki/Limits#Defense.2FToHit
Generally how it works. Buffs and Debuffs modify it further.