I would classify them as:
Phase 1: 3-5 people from each team collide, CC is popped people are starting to build up their combo's, the AD carry might die in this phase already.
Phase 2: Any people who were not at phase 1 will be present now (or not) all the burst is being unleashed, people die and this is where skill comes in to play.
Phase 3: (cleaning up) The winning team may chase the last 1 or 2 remaining champions from the enemy team (or let them run).
Give your description (and possibly more phases, depends on how you like to word it)