Cement is commonly sold at building material dealers in 25 kg bags or in bulk by weight.
There are several types on the market marked by a number ("325" - "425" - "525") which indicates the resistance in Kg/cm2 after 28 days.
The most widespread is 325. Cement bags have an internal protection against humidity but it is however a good rule to keep them in a dry place, possibly not on the ground but on a pallet.
Expiration of cement:
Not everyone knows that cement has an expiration date, in fact after a certain period it can no longer be used, on pain of a drastic reduction of its mechanical characteristics.