A commercial pilot license (CPL), is a qualification that permits the holder to act as a pilot of an aircraft and be paid for their work.
The basic requirements to obtain the license and the privileges it confers are agreed internationally by International Civil Aviation Organization (ICAO). However the actual implementation varies quite widely from country to country. According to ICAO, to be eligible for a commercial pilot license, the applicant must be able to read, speak, write, and understand English: already hold a private pilot license, have received training in the areas of a commercial pilot, and successfully complete the relevant written exams. To proceed in obtaining a commercial pilot license, the applicant must first obtain Class I medical certificate. Upon completing those prerequisites the applicant will then receive an exam from the governing aviation body that consists of an oral and practical flight test from an examiner.
A license will contain a number of sub-qualifications or ratings. These specify in more detail the actual privileges of the license, including the types of aircraft that can be flown (single-engine or multiengine), whether flight under instrument flight rules is allowed (instrument rating), and whether instructing and examining of trainee pilots can be done (instructor or examiner rating).
A Commercial Pilot License (CPL) is what you would need to get a job, and get paid as a pilot. If you want to join the airlines as a pilot, this is the minimum requirement. You will need about 200-250 hours total flight time to get your CPL.
Just like your Private, you would need to pass a written and a practical exam to get your Commercial Pilot License.