Can someone tell me where I can find answers to these questions...
1) As far as I understand everything I'll do with BITCOIN will stay in the folder of the application. This means that if I buy or sell bitcoins all files necessary to get the info will stay there. So this means that (GOOD) keeping that folder on a USB is like having cache and (BAD) loosing that folder I loose everything (no user/password system) .. Correct?
2) Can I create a folder on a MAC machine and then use it on a PC machine? Or iPhone? etc...
I think that should work without any problems. The devs might be more sure about this.
3) My folder is called "BITCOIN-0.3.19" is it this one we're talking about ?
Well, it's just a name, so I guess so. The files you want to save are in your profile or home directory.
You might want to upgrade, there is a bitcoin-0.3.20 already out.
4) Wich is the file I shouldn't never ever ever loose.. the one that contains all info on my transactions?
wallet.dat contains all the cryptographic keys to your coins. That file is what you want to keep.
Transactions are not in that file and can be redownloaded through the network.
5) How do actual transactions occur ? I go to an online shop and want to buy X. Instead of clickin on CreditCard or Paypal I click BitCoin.. then?
Implementations can differ between stores, but it will go something like this:
1) Go to shop, select product and go to a checkout site.
2) You will be presented a bitcoin address.
3) You transfer the required amount to that address (from within your client).
4) Transaction completed and your product will be shipped.