I take it you're using D&D terms. In that case - a fast cast spontaneous spell is somewhat similar to an Interrupt action.
Fast casting is like an Interrupt in that you can take it at any point in the round, outside of your turn, and it can interrupt another's action (if the Finesse roll is high enough; generally above his Initiative Total).
Note that since a magus "cannot cast ... a fast-cast and a normal spell, in the same round", if you fast-cast before your turn arrived in the round you can't cast another (non fast-cast) spell in that round, whereas if instead you already cast your normal (non fast-cast) spell in this round then you can't fast-cast a spell in it. This is unlike the Interrupt, that (if I remember correctly) revokes your next major action.
Cheers,
Yair