agi / ac questions

[Donruss]

Agi gives ac under your hps and mana... does agi make you harder to hit? Like they miss more? Or does it add to your ac total so ac you take less damage? At lvl60 will a priest get better returns with ac or agi? I feel like a Djarns ring would give me more ac than the 5ac 55hp rings, am I right? I obviously have no understanding of how these things work.