Everything that is pre-installed on the iPad is free : http://www.apple.com/ipad/built-in-apps/
You can also download a copy of the iPad manual from here which has brief chapters on each of the built-in apps (plus iBooks which is a free download) : http://support.apple.com/manuals/ipad/
In terms of when you will be charged, then that will happen when you click on 'buy' on an item in the store (they are charged immediately). Some apps also allow you to make in-app purchases within them (though you can prevent them being possible by setting restrictions on the iPad via Settings > General > Restrictions : understanding restrictions). Some apps also allow you to take out auto-renewing subscriptions within them which are charged automatically each week/month/year until you cancel them e.g. newspaper and magazine apps.
Note that whenever you change the credit card details on your account you may see a temporary store holding charge on it, which should disappear within a few days - so if you've just created your account don't be surprised if you see a $1 'charge' on it for a few days
They charge your credit card when you buy something from the store, unless your account has credit applied to it via the Itunes or App Store gift cars that can be found everywhere. Then they'll deduct the cost of the purchase from that credit rather than charging your credit card.
You will also receive en email invoice stating the charge.
You should know when you purchase something as you have to click the Buy button, and the price is displayed prominently under App picture in the store.
The iPad comes preloaded with :
Safari Web Browser, Clock, Calendar, Notes, Camera, FaceTime, Maps, iMessages, Contacts, Reminders, iBooks,Newsstand, Photobooth, Mail, Photos, Videos, Music, Itunes, and App Store Apps.
There are many many apps that are free as well that are not preloaded, but can be downloaded at no cost.