Charter, also called air taxi or ad-hoc flights require certification from the associated country's regulating body such as the FAA in the U.S. The regulations are differentiated from typical commercial/passenger service by offering a non-scheduled service. In the U.S. these flights are regulated under FAA Part 135. There are some cases where a charter operator can sell scheduled flights, but only in limited quantities.
No matter what company you're flying with, be sure to ask if there two pilots or one. (Though two pilots are standard on commercial flights, regulations vary for private planes of all sizes.) It also helps to ask if the operator owns the plane—typically, companies that own their planes offer better service. Never forget to ask about daily minimums and taxi fees. "If the hourly rate is $9,000 and you have a single 40-minute flight, you might assume you are going to pay $6,000," says Doug Gollan, creator of PrivateJetCardComparisons.com. "But if your provider has a daily minimum of 1.5 hours, you are going to actually be charged $13,500, plus taxi time, which in my comparisons I have found varies between being included and up to 12 minutes per segment."