This is the highest license the FAA issues:
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.
Before we carry on with flying, please be aware of the following:
Compared to riding cars in the United States, you are ....
1) ... approximately 7 x more likely to die in a General Aviation airplane per mile traveled.
2) ... approximately 3.5 x more likely to die in a General Aviation airplane per hour spent on the plane.
3) What I will teach you is to MANAGE those risks, by giving you the relevant tools! You will lessen the risks stated above considerably through training, knowledge, and practice.
In this regard, flying is no different from scuba diving, renting watercraft, or even skiiing! You must exercise prudence and caution!
A good comparison is getting a ride from your 55 year old mother vs. driving with your 19 year old son. Insurance rates bear out this assertion. The risk differences are considerable.
Remember that pilots, too, are not a homogenous group, and there is no reason why you should not be able to bring the odds down to "car-levels."
You will not regret having learned to fly!