To begin with, only Armed Guards will have Tasers. Tasers can also be used by normal guards by purchasing the 'Taser Rollout' within the Bureaucracy system. Once bought, each guard must then pass the Guard Taser Certification program. Once a guard has passed this program they will be able to use Tasers, although for each guard that passes the program you will be charged £400 (the cost of the taser). Guards must then visit the Armoury and pick up their taser.
Tasers are very efficient to stop up to a few prisoners in a small riot, or in an escape, but has too slow of a rate-of-fire to be effective in a large riot (note small riots of 10-20 prisoners are an exception). They will immobilize your prisoners for a pretty long time (about half an hour to an hour in-game)