Cheap, no frills. You have to pay extra for all the extras: seat choice, checked luggage, etc...
If you plan to use them even twice in the same year, join their club (the Den). The annual fee for that is easily offset on the discounts. Also, you get deals for buying the extras at a discount including the extra room seating.
If it's a local airport, then you save a lot on travel time and parking. With my local airport (Trenton NJ), the security checkpoint doesn't even open until 45 minutes before the flight. None of this "get there two hours early" stuff.
These small carriers thrive on having a fleet of the same, new moderately-sized planes which makes for a lot of efficiency in their upkeep and planning.