I've been using the Serebii B&W Pokedex for a while now and found out that almost every Pokemon now has a 'hidden' ability? Are these abilites hidden as in they are not announced in the dialog in battle? Or do you have to obtain them in some way? Do Pokemon have the 'hidden' ability at the start? Can they be used in any sort of battle (Subway etc)?
Example: I have an EV trained Politoed, and it's 'hidden' ability is Drizzle (just like Kyogre It causes rain). My politoed entered battle, but it didn't start raining.
Please answer all of my questions above, and give some more insight about the topic. Thanks