[Vedran]

Something from Player's Handbook:

Wizards cannot wear any armor, for several reasons. Firstly, most spells require complicated gestures and odd posturings by the caster and armor restricts the wearer's ability to do these properly. Secondly, the wizard spent his youth (and will spend most of his life) learning arcane languages, poring through old books, and practicing his spells. This leaves no time for learning other things (like how to wear armor properly and use it effectively). If the wizard had spent his time learning about armor, he would not have even the meager skills and powers he begins with. There are even unfounded theories that claim the materials in most armors disrupt the delicate fabric of a spell as it gathers energy; the two cannot exist side by side in harmony. While this idea is popular with the common people, true wizards know this is simply not true. If it were, how would they ever be able to cast spells requiring iron braziers or metal bowls?
For similar reasons, wizards are severely restricted in the weapons they can use. They are limited to those that are easy to learn or are sometimes useful in their own research. Hence, a wizard can use a dagger or a staff, items that are traditionally useful in magical studies. Other weapons allowed are darts, knives, and slings (weapons that require little skill, little strength, or both).

It says little about priests and armor, but their spells are effectively prayers, they don't have to be highly mobile as wizards do.