The only rule is that a ball must reach the rail after contact. It can be the cue ball or an object ball. There are many different breaks you can use, although the two most common are detailed by Sean earlier in this thread. Play around with it and see what you like. Most employ the "traditional" break, but others can be effective.
I've seen players use a straight pool safety break before, as well as hitting the stack hard on a multitude of occasions (although unless you're Corey Deuel, this one doesn't usually turn out well).