You need an ATP whenever you are flying as a captain (or "PIC", which means Pilot-in-Command)
1. either jet-aircraft, or
2. aircraft weighing in excess of 12,500 lbs
Most people who set out to become professional pilots initially instruct, and when hitting the "magic" 1,500 hour flight-time mark, take their written, oral, and flight-exams to get their "ATP."
It is a very exacting exam, and not in the least easy, considering that the typical multi-engine ATP test is conducted in small piston-engine airplanes, such as a Piper Seneca.
The fact that these types of airplanes are relatively underpowered (compared to a jet-aircraft), and typically under-equipped (i.e. no auto-pilot), requires the pilot be particularly good at energy management (among many other things).
If one can pass their ATP on a small multi-engine airplane, they should have few problems from a flying point-of-view to upgrade to a larger airplane.
What are the requirements?
Besides a Commercial Pilot License & Instrument rating, along with a Class I medical (not needed for the check-ride, but for a job down the line), you need 1,500 flight hours total time with certain amounts apportioned to night- and cross-country- flying.
However, get the ATP on a multi-engine airplane, as the single-engine version provides you with practically no job prospects.
