Career path

Odyssey uniform - Department service colors — Starfleet uniform code for the three departments

In Star Trek Online, players may choose one of three career paths (sometimes also classes, previously called "professions") they wish their character to follow (Tactical, Engineering, Science). Although originally based on Starfleet's departments, players of any faction have to decide for a career path during character creation. Like the player's race, a career path is a permanent choice that cannot be altered later in game.

Although each career path offers certain bonuses towards specific play-styles in space and ground combat (e.g., damage dealing, or healing, or robustness), character roles are not very rigidly defined in Star Trek Online, and any character may fly any ship, wear any uniform, and equip almost any item (the only career-specific items being kits). Although Bridge Officers are likewise categorized into the three career paths, they solely determine the pool of abilities they have access to.

In general, career specific elements in Star Trek Online encompass:

Kits: Kit frames & modules are career path-specific, equipable items granting the player ground abilities.
Ground skills: The ground portion of the skill tree is specific for each career path.
Career-specific traits: A set of four special traits is added to the pool of selectable traits depending on the player's career path (see below).
Career-specific abilities: Each player is granted a set of ten career-specific abilities, five for space, and five for ground combat.
Career-specific craftable Training Manuals: Depending on the career path as well as allocation of skill points, a player can craft a specific set of Training Manuals, which can teach Bridge Officers of the same career path new abilities; some of these manuals are not obtainable otherwise.
Career-specific mission objectives: Certain missions in the game offer career path-specific objectives (a purely cosmetic "gimmick").

Tactical

Tactical officers are most useful in the DPS or Damage roles; they do their best work while dealing damage to enemies in ground and space combat. Tactical officers usually wear red and are best matched with Escort vessels (Federation) and Raiders or Raptors (Klingon Empire), which are also the best starships in terms of inflicting damage to enemies. They do this through maximizing the damage output of their Energy (Beams & Cannons) and Projectile weaponry (Torpedoes & Mines), and through increasing that potential through Attack Patterns (α, β, Δ, and Ω).
In ground combat, tactical kits give powers that allow the player to put out increased damage as well as spike damage (one-time concentrated bursts of firepower), AoE (Area of Effect) damage via grenades, and abilities that improve the damage output and staying power of allies.

Unique traits

Crippling Fire (Space): Your Critical Hits reduce your target's Accuracy.
Last Ditch Effort (Space): During Go Down Fighting, +Res from attacking
Situational Awareness (Ground): Grenade powers you use will expose their targets.
Strike Team Specialist (Ground): Attacking targets gives +Crit Chance

Engineering

Engineering officers are best suited to the Healer and Tank roles. Engineers wear the color gold for their uniforms and are ideally matched with Cruisers (Federation) and Battle Cruisers (Klingon Empire), which are also best suited for tanking. Tanks take the brunt of damage in battle and try to attract all enemies to themselves, while the DPS or Damage players do the majority of the actual firing and damage. As healers, Engineers are adept at healing hull damage, although they also have strong shield healing. Their focus is primarily on survivability and resilience in space combat, while supporting their Away Team in both space and ground combat with fixed-position automated weaponry, shield and medical generators, technological buffs, and other repairs and improvements.
In ground combat, engineering kits generally involve fabricating turrets and shields, supporting your team with weapon buffs and seeker as well as support drones, and laying mine barriers and bombs.

Unique traits

EPS Manifold Efficiency (Space): Batteries give a secondary boost to all subsystems.
Grace Under Fire (Space): Taking damage can reset the recharge on Miracle Worker.
Nanomolecular Architect (Ground): Turrets & drones gain health regeneration and increased damage resistance rating
Shield Harmonic Resonance (Ground): Being attacked gives you +Res

Science

Science officers are most useful in the Healer and "Buffer/Debuffer" roles, where they provide healing and buffs for allies (and themselves), as well as powers that weaken and disable enemy NPCs. Science officers wear the color blue and are ideally matched with Science Vessels (Federation) and Support/Science Vessels (Klingon Empire), ship classes whose bonus is to weaken and control their enemies, and provide some healing on the side.
In ground combat, science kits offer team healing and AoE (Area of Effect) damage, but can also grant abilities that control or debuff enemy resiliency, damage and movement.