Credit cards can be used only in big hotels and at car-hire companies. You'll need dinars for day-to-day expenses, although tourist-oriented businesses (hotels, airlines, tour companies etc) will often accept euros.
The black-market exchange rate is significantly better than that you'll get at banks. Ask locals where you can change money (it's considered routine, if not strictly legal). You can only change dinars back to euros or dollars unofficially.
Some Algerians, especially in rural areas, might give prices in centimes rather than dinars (there are 100 centimes in a dinar). To confuse matters further, they might also drop the thousands, so a quote of '130' means 130,000 centimes (ie DA1300).