Semen is stored in liquid nitrogen. This is about 320 degrees below zero. This is the only way it can be stored. You need a special nitrogen tank to store semen or contact your local inseminator to see if he will store it for you.
The other responses are right on, but to add to them, semen can be stored indefinately in liquid nitrogen tanks. I have one vial(the old way semen was contained) that was collected on a bull in 1972. It is still viable as we used one a few years back and have a daughter of the bull in the herd producing.
Jason Trowbridge Southern Angus Farms Alberta Canada