1. Summit : Summit is the meeting of the heads of the govt.
Ex: PM Manmohan leaves to attend BRICS summit; and summit has a 'chairing system'. Every meeting is chaired by a country or a person.
Ex: 21st ASEAN summit was chaired by Cambodia.
2. Conference :
Conference is not necessarily the meeting of the heads of the govt. Anyone (obviously prominent persons only) can attend the world conference on behalf of thw govt. or a country.
Ex: World Religion Conference , held in Chicago , headed by Swami Vivekanand representing India.
Rounds are the frequent meetings on the same topic again and again.
Ex: WTO / GATT rounds.