The prime/subprime dividing line can be anywhere between 620 and 660, depending on who's creating the definition. Most of the time, it's usually set at 620 or 630. Oversimplified, a credit card that can be approved for applicants with a FICO score below 620 would be called subprime; a credit card that requires a minimum FICO score of 630 or higher would be called prime. Subprime credit cards typically have annual or monthly fees, low credit limits, and high interest rates. But the definition is a bit murky, and there are a number of credit cards that could be described either way.
