Star Trek's class system is very loose compared to normal MMO's. It's not like you pick X and and you are a Tank or a healer. There is also two types of class system in the game.
The first is your character class. This is chosen at the start of the game and can never be changed. Your character can be either a Tactical, Science or Engineering Officer.
- Tactical Officers
Tactical Officers focus on offense related skills and buffs, both for themselves and teammates. In a traditional MMO sense, they are the "DPS" class.
- Science Officers
Science Officers focus on support related skills and buffs, both for themselves and teammates. In a traditional MMO sense, they are the "Support/CC" class.
- Engineering Officers
Engineering Officers focus on defense related skills and buffs, both for themselves and teammates. In a traditional MMO sense, they are the "Tank" class.
The second type of class is your ship class. There are Escorts, Science Vessels and Cruisers.
Escorts are designed to be fast, maneuverable warships with emphasis on the ability to deal heavy damage. They also have access to the Dual Cannons weapon type. All this damage does however come at a cost: they have the weakest hull and shield strength of any class. Escorts are the best type of ship for Tactical Officers, due to having more Tactical Bridge Officer and Tactical Console slots.
- Science Vessels
Science Vessels are the support vessels of the fleet. They have the strongest shield strength of any ship class, but have less weapons than the others too. Science Vessels are the best type of ship for Science Officers, due to having more Science Bridge Officer and Science Console slots.
Cruisers are the largest and most resilient vessels in the fleet. They have the strongest hull strength of any ship class, as well as the largest crews, and power supplies. They also get more Device Slots for extra shipboard items (such as deployable Turrets and Torpedo Platforms) than other classes. Due to their size, they turn very slow and are difficult to maneuver. This can make it hard to bring your weapons to bear on the enemy. Cruisers are the best type of ship for Engineering Officers, due to having more Engineering Bridge Officer and Engineering Console slots.