Pokemon Black and White 'Hidden' Abilities?

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

Pleaselog inorregisterto add a comment.

These hidden abilities can only be obtained through a wifi system known as thedream world.In addition to abilities, you can get things like rare items in the dream world. The link will explain a bit more, but almost every pokmeon, except for specific one like Shedinja or Slaking, have alternate hidden abilities, also referred to as dream world or DW abilities. They threw them in there to update a few of the older pokemon that deserved better abilities as well as shake up the gameplay a bit.

Hope this helped.