Crates are special time-limited offers for which one can acquire new, powerful units with Volt Coins.
A crate contains a number of possible units (usually only one of the most powerful unit, and many copies of the least powerful units). Each time you open a crate, you get one of the units left in the crate. (Thus, if you buy all of the contents of a crate, which typically requires many buys, you are guaranteed to get one and exactly one of the most powerful unit in the crate, along with a lot of junk.)
Some crates will first appear when you get a particular mission (like the Laser Crate), while others will appear when they become available.
Some crates contain a mix of crate-exclusive units and normal premium units, while other crates contain only crate-exclusive units.
List of crates: