The promotional period on a 0 APR Card is determined by two criteria.
The two criteria, one or both of which may be offered on 0% promotional terms are balance transfer(s), or credit card purchases within a set period of time.
The period of time the interest free period will cover will vary, usually between 6 and 12 months.
A balance transfer, like the name suggests is where debt that is owed by you elsewhere - typically on one or more other cards with a high interest rate, is transferred in part or in full onto the account balance of the new credit card.
However the card issuer will likely charge you a small percentage fee for each transfer performed.
So this cost needs to be factored in to ensure there is still a significant advantage to be gained from using this facility on your 0 APR card.
Credit Card Purchases, like the name suggests occur when you use your credit card to pay for goods or services instead of cash.
Typically this is used when large purchases need to be made to provide a convenient way to spread the payments over several months interest free, The 0% promotional period on a card will be offered for balance transfers, or purchases.
It may also be offered for both, however the duration of the interest free period may differ between the two - be sure you're fully aware of what is being offered.
A 0 APR card should not be used for managing debt that you are unable to comfortably clear in full before the end of the interest free period.
Remember interest is being accumulated throughout the term of the promotional period.
Ensure your balance is paid in full before the end of your promotional period so you do not pay any interest.
Also ensure you never miss a payment deadline, as this could lead to the card issuer immediately terminating the interest free promotion as well as harming your credit rating.
You would also be wise to read over the card issuer's small print to be sure you fully understand their terms.