Crews in World of Tanks Blitz are much different than their counterparts on the computer version. In Blitz, it is said that you have one crew that commands each and every tank. This crew's efficiency in a certain tank is measured by the crew %. There are no individual positions (commander, gunner, driver, etc.), but instead one whole, functioning crew, in the form of a percent value. In addition, the crew cannot be transferred from tank to tank like in other Wargaming games. Your crew on your tank is identified solely by their crew skill (which ranges from 50% to 100%)
When you obtain a vehicle, you will receive a set of crew with 50% training. You can either train the crew to 75% using credits, or 100% using gold. Many players use gold to train their higher tier crews straight to 100%, as the amount of XP needed to train your crew to 100% grows exponentially as you move up the tiers, whereas the price (in gold) only goes up in a linear trend. The benefits of having 100% crew is that, when compiled with equipment and provisions, your reload time is at its absolute minimum. Therefore, you can accurately gauge peek-a-booms/trading shots when facing off against the same tank, the stats of your tank increase by a substantial amount, and you can start training crew skills immediately. Selling the tank will not dismiss your crew. When you buy the tank back, you can get back to the crew qualification level when you sold the tank.
For example, if you buy a tank, play battles in it and get it trained to 88% and then sell it, when you repurchase that same tank, it will have an 88% crew.
Premium Tanks have 100% crew included. There is no need to train, pay credits or gold to upgrade the crew. All crew xp earned in premium tanks goes towards account Crew Skills.