Crew

Crew are the people who navigate, pilot, fly and command your spaceships under your commands. They are ordinary inhabitants who have been trained to be able to do the above skills; as such they are more valuable then inhabitants. Important to note is that they are measured in 10's, whereas inhabitants in thousands. Crew are trained on worlds in crew training centres which are buildings within worlds. Crew can also be purchased on contracts from other empires, but this depends on your empire morale which directly affects the price, and indeed, if any crew will work for your empire.

Boarding
Crew men are used to board and defend against enemy boarding attempts. Your Crew-Combat-Level determines their effectiveness. If your crew combat level is higher then theres and you have more crew you will win the engagement and be left with control of the ship you invade. If your crew combat level is higher, and you have less crew you can still win the engagement, depending on the offsets of their level, the difference between the crews. To read more on boarding, see the full article.

Requirements for systems
Most systems require only basic amounts of crew: around 5 units (50 actual) of crew will be able to perform basic tasks on the ship such as replication of systems, movement/flight, energy generation and cargo storage /transferring , whereas active roles like combat require around 80% of your crew, and the effectiveness of the ship in said combat is proportional to the percentage of that 80% which is available. If you only had 40% of the standard crew, or 50% (half) of the required combat crew, your ship would only be able to display combat skills to about 50% of its ability when up to the standard crew rate. This means without crew, your ship suffers serious drawbacks, and under 5% crew, or 50 units [500 crew], which ever is smalles, your ship is unable to perform combat actions.