Workers
Workers are laborers that you assign to various places, such as your capital, mines, or research labs. Typically they are either humans from your crew, which you get by successfully inviting friends, or droids which you build at the droid factory.
Humans are usually preferable to droids, as they are free (all it takes is an invite) and more productive. However if you don't have any friends who will accept your invites, droids at least enable you to increase your production.
You are allowed one worker at level 0, and then another 1 worker for every 3 levels. So for instance a level 3 Ore Mine can have 2 workers, a level 6 Ore Mine can have 3, and so on.
Locations
These are the types of workers you can assign to each location.
Research Lab
Droid: Lab Droid
Capitol
Droid: Build Droid
Mines
Droid: Mine Droid
Crew
The Crew Droid gives you one crew point per unit, but humans invited from the Crew Page give you 3. Crew points are required to run missions.