Many cards offer periods of intro 0% APR before the regular APR kicks in. A card might have an intro APR period on balance transfers, purchases or both. A balance transfer card is simply a credit card that offers an intro APR on balance transfers. It's also possible to find cards that charge no balance transfer fee, though they're relatively rare.
When considering cards with interest-free periods on purchases, note that a credit card with an intro APR offer is different from one offering deferred interest. In the former case, you'll start to accrue interest on any balance you're carrying after the intro period ends — but in the latter, you'll owe interest on the entire amount from the date of purchase if you don't meet your card's deadline to pay off the debt. Be especially cautious of deferred interest with store cards, where such offers often appear.