discipline = grenade cooldown, int = super power cooldown, strength = melee special attack timer for all 3 classes I believe. On your character sheet if you mouse over the stats on the right side it'll tell you the current effect on you so you can swap pieces of gear and see.
For PVE I think most will want to favor disc over the other two stats, but purples always have two stats (str/dis, int/dis, or int/str - i always go int/dis). I don't think str is useful that much for PVE since the times you can run up and punch something is ... small. However for PVP its a good stat, although it sorta varies more with the PVP map and your PVP play style (long range, flag protector, etc).
the other thing to keep in mind is that purple/exotic gear have different bonuses, like more ammo for X weapon or faster reload times. So the exact same armor can have very different bonuses and it will vary every day from vendors (or every engram), so ideally you'd want one that matches your mostly used weapons. In PVE I usually use handgun and sniper (ice breaker) so I look for armor that boosts that, for PVP I use suros (auto) and shotgun. Its a bit of a pain having to constantly reswitch all my gear depending on play mode and activity, wish there was a gear set manager.
The most important thing is getting to light level 28 though, which is an exotic fully upgraded, and 3 purple armor pieces upgraded to everything but the last tier and one upgraded once or twice in that last tree (98 or 99 light I believe?). Being BELOW an enemys level puts you at a severe disadvantage, but being higher than them doesn't really do anything.