1) Every Messenger is a prophet, but not every prophet is a messenger. A prophet is one chosen by Allah, receives revelation or divine inspiration, and is infallible. A messenger is a prophet who also has an official message to his people (like a new religion or scripture). Many prophets did not have an official message. They were just sources of guidance to the people.
2) The Ulul-azm Rasouls are very distinguished prophets who have the highest level of faith and willpower. They are the greatest of messengers and the most high ranking.