AVs max at 200 per stat. There's two main kinds of candies, stat-specific ones, and species-specific ones. The stat candies come in small, medium, and large varieties, and boost their respective stats, but only up to 100 for small and 150 for medium. Species candies have no restrictions to how often they can be used, and they boost all 6 stats at once, with the downside being that they're more specific when it comes to who they can be used on, and how you obtain them in the first place.
As for how they're different from EVs, EVs only gave you one stat point for every 4 EVs you had in that stat, whereas AVs are a 1:1 ratio, so if you, for example, max out the attack AV of a Pokemon that usually has an attack stat of 150, they'll now have a ridiculous 350 attack. This might seem like it would make certain Pokemon have an advantage over others, but all Pokemon can have all 6 of their stats pushed up by 200, meaning that in the end they're still the same strength relative to each other, but now they utterly curbstomp NPCs.