From UFStarfleet Wiki
Vital Missions: Also known as Priority One command, a Priority 1 mission supercedes all other mission types and overrides all orders and regulations under General Order Four. The fate of the Federation relies on this mission, and all Starfleet lives and ships are considered expendable for its completion.
Urgent Missions: Urgent missions usually involve rescuing or protecting the lives of thousands, if not millions, of Federation citizens. A ship carrying the cure for a plague threatening to wipe out large populations undertakes an urgent mission. Typically, only vital missions supercede these.
Strategic Missions: These missions usually involve securing or defending resources or Starfleet outposts. Missions of high threat factor during peacetime, such as patrolling the Romulan Neutral Zone, are Category C missions because of the high probability of danger, as are escort duties.
Routine Missions: Encompassing the majority of Starfleet missions, a starship crew may undertake dozens of routine assignments at the same time: categorizing gaseous anomalies, measuring pulsar fluctuations, researching the life cycle of the Gamelan root beast, conducting soil analyses, and putting in an appearance at Caldos Colony, for example. Routine missions include most patrols, exploration, and research duties.