Yes they are separate, debit uses my bank account directly and credit is invoiced every month separately.
The selection process is very simple, you just put your card into the machine (almost all the cards nowadays are chip based nowadays but there is also a magnetic reader usually on the right), something like this
I'm an Indian living in the UK and don't have this figured out yet. What's the difference when you select to pay by credit, if the balance is paid automatically from your account at the end of the month, instead of being debited immediately from your account? Or do you have to manually pay back your balance?
Well, there's the obvious difference, that the money stays in your account for longer with a credit card.
There are legal differences, too: credit cards are regulated by the Consumer Credit Act, which makes the card issuer jointly liable in many cases. So, for example, if you order a product with a credit card, and the retailer goes bust before supplying it, the credit card issuer is also liable. If you'd bought it with a debit card, you'd be out of luck.
Finally, if your card details are compromised, you've slightly longer to sort things out with a credit card: with a debit card, you could find yourself with no cash at all.
The selection process is very simple, you just put your card into the machine (almost all the cards nowadays are chip based nowadays but there is also a magnetic reader usually on the right), something like this
http://data.talka.com/files/Yomnani.JPG
then the machine shows amount and you select
1) Credit
2) Debit
Then it asks for pin and that's it.