The Vow of Poverty is an feat found in the Book of Exalted Deeds. It is an Exalted feat, (i.e. only Exalted characters can take it), that is one of everal different sacred vows. Each of the Vows is a type of vow that an ascetic might take. For instance, there are also a vows of chastity, non-violence, etc. Each of these vows is intended to stretch the limits of what a D&D character can be.
Most of the other vows grant some bonuses to different types of saves. The Vow of Poverty, is the most powerful of them. It grants a variety of different benefits from an AC bonus, to save bonuses, to stat bonuses. It mechanically recreates the effects of wealth without using wealth.
The advantages of this are that the Vow's effects cannot be dispelled. Furthermore, the takers of the Vow tend to have higher AC's than party members.
The disadvantages are pretty obvious. The player can never use any magical items. The druid in the party the only one left to use the Staff of Life to resurrect the Cleric and the Rogue? You're up a creek. The character is also limited in versatility. He cannot expand his abilities through the use of magic. Furthermore, the character is useless in ranged combat. (Except for the druid casting Call Lightning.) Lastly, even though the writers of the Vow attempted to prevent this from happening, a taker of the Vow just isn't as good as a member of the same race/class/level as one who didn't take the vow in killing things.
That's the Vow of Poverty in a nutshell. Any other questions?
Sparxmith