Defense is basically your Avoidance stat: Chance to Parry incoming Melee (lightsaber, boss fist, monster teeth) attacks and Dodge/Deflect incoming Ranged (blasters) attacks. Shield / Absorption are your damage mitigation stats. Mitigation is always more reliable than Avoidance. Tanks who stack Avoidance over Mitigation take more unpredictable spike damage, whereas tanks who stack Mitigation over Avoidance take consistent but predictable damage. You want a mix of both, but definitely more Shield/Absorb than Defense.
Example*: A tank with 50% Avoidance and 20% Mitigation will either take no damage or a very large hit (aka spike damage). Situations like this are very stressful on healers. A tank with 20% Avoidance and 50% Mitigation will take an average of half damage on all attacks, and a small chance to take no damage. This is what healers prefer, as they can easily keep up with the incoming damage since it's more consistent. (as opposed to: fine, fine, fine, wtfpwn tank dead lulwhut?)
All of the defensive stats only work on Melee and Ranged attacks, which is fortunately the majority of PvE boss attacks. None of them work vs Tech (grenades, vibroknives, probes) or Force (self explanatory...) attacks though. So you can think of Shield/Absorb as an extension of Armor since the vast majority of Melee and Ranged attacks are Kinetic/Energy dmg.
Shield and Absorb cap at +50%, for a total of 70% shield chance and 75% shield absorption. Defense caps at +30%, for a total of 40%. IMO, you'd want Shield/Absorb on your Enhancements in your main gear and Defense Rating on your Mods / offpieces (ear, implant, belt, bracers). Once Shield/Absorb are at or near their caps you can put more focus onto Defense. And if you somehow manage to cap off all of those... uhhh, stack Power?
*Note: because Shield Chance only caps at 70%, no matter how you stack stats you're still going to get hit with full damage from time to time. but it's still better than relying solely on Defense.