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Obtaining a Commercial pilot certificate allows you to fly for compensation or hire. Whether you want to fly for the airlines, become a charter pilot, fly cargo, or provide flight instruction, becoming a Commercial pilot is the first step in this process.
It is most common that you obtain your instrument rating prior to starting commercial pilot flight training. A commercial pilot without an instrument rating will have the limitation "The carriage of passengers for hire in airplanes on cross-country flights in excess of 50 nautical miles or at night is prohibited." (FAR §61.133)
If you don't already have a complex endorsement, this will be part of the training. A complex (flaps, controllable pitch propeller, and retractable gear) aircraft is required for the practical test.
What you can do as a Commercial Pilot
Note that what you can and can't do is complicated as a Commercial pilot. You are not able to "hold out", as in advertising your services along with providing an airplane, for the purposes of flying cargo or passengers. Operations such as these fall under the regulations in parts 119, 121, 125, 129, and 135 and have further requirements. For example, if you want to run a charter or "Air carrier" service, you will need to do that under a Part 135 certificate. You can work as a Commercial pilot for a Part 121 airline, but you can't be your own airline with just a basic Commercial certifcate. The jobs you can do with a basic Commercial Pilot certificate include:
Sight-seeing flights within 25sm
Ferry Flights
Crop dusting
Aerial photography
Banner towing