Your population grows and diminishes as you progress in the game. You receive new free colonists every tick based on your current population, as long as you have enough food to feed all your colonists. You can build farms in the resources view to get additional food and the Biodomes planetary project improves these farms.
The bigger your population, the faster it grows. But you can also build hospitals and maturation chambers to speed up colonists growth. Each level of the building is more expensive than the previous one, but so are the farms! A good tactic is not to exhaust your population in a single order, unless your really need to!
There are five different trained colonists to help your colony;
Technicians are required to build facilities and buildings as well as to construct planetary projects.
Miners control your ore- and fuel-miners to gather credits and fuel for your colony.
Each farmer controls one farm to produce food for your colony.
Pilots are needed to man your spaceships and need to be trained before ordering your ships.
Soldiers are required for any ground units you build as well as some CDS require soldiers.