a single stage ak trigger is a very safe, long travel device with only one take up/let off geometry designed into it. it has a relatively heavy, constant pressure (pull weight) requirewd to cause the rifle to fire at the point the hammer releases. aftermarket single stage triggers are usually lighter in the pull weight department than military triggers.

a two stage trigger has a relatively heavy pull weight to the first stage. the second stage will be shorter in travel and much lighter at the point the shot is released.