Hello Tank Commander,
All tanks must have a crew to enter a battle.
The crew can be recruited at the following levels, 50%, 75% and 100%. Once a crew member has reached 100%, they can learn skills and perks that allow them to have an advantage on the battlefield.
Perks can be used after they reach 100% trained, whereas skills start working right away.
Skills only reach their full potential once they reach 100%, and for every four skills/perks, the amount of Crew experience needed to reach 100% skill or perk doubles.
Some crew also have unique skills. We call such a crew a "Hero crew". Bear in mind, you can only receive these perks with the crew, and should you decide to retrain your crew member, you will lose this unique skill. Please be especially mindful, since Player Support has no means of reversing this action.
On the other hand, all base Crew experience received is the same amount as the base Tank experience you receive from a battle.
We hope you found this explanation helpful.