A fleet in Star Trek Online is a group of players that have banded together. Fleets have a standard command structure and management tools commonly found in other MMORPGs for guilds, e.g., a rank system, a fleet bank, a fleet specific chat channel, a calendar, and - most importantly - a Fleet Advancement System with different fleet holdings.
Fleet Advancement System
With Season Six: Under Siege, a dedicated advancement system for fleets was introduced. It allows the fleet to build and develop various fleet holdings:
|List of Fleet holdings|
|Image||Name||Departments||Tiers||Purchasable item category||Location|
|Fleet Starbase|| Military
|5|| Ship Weapons, Ship Shields
Deflectors, Impulse Engines
Ground Weapons, Personal Shields, Elite Carrier Pets
|Eta Eridani Sector Block|
|Fleet Embassy|| Recruitment
|3|| Threat-Scaling Science Consoles
|Fleet Dilithium Mine|| Development
|3|| Fleet Warp Cores/Fleet Singularity Cores
Advanced Engineering Armor Consoles/Advanced Engineering RCS Consoles
|Beta Ursae Sector Block|
|Fleet Spire|| Research
|3|| Plasma-Integrated Warp Cores/Thoron-Infused Singularity Cores Advanced Tactical Vulnerability Consoles, Spire Carrier Pets
Spire Experimental and Prototype Kits
|Solanae Dyson Sphere|
Increasing the tier of holdings is achieved by completing projects within their respective departments. As all fleet members can contribute, these projects require large amounts of various resources, first and foremost fleet marks and refined dilithium .
Furthermore, as a fleet develops its holdings, it gains "Fleet XP", thereby increasing its "Fleet level". This is a purely cosmetic value, comparable to personal Accolade points, designed to quickly show the overall experience, age, and progression of a fleet. For reference, having reached the maximum tier of the starbase, the embassy, the dilithium mine and the spire (including all their respective departments), a fleet will have achieved level 20.
Other fleet-specific functions
- See also: Banks in Star Trek Online
The fleet bank is a universal storage facility for inventory items and credits that is accessible by all members of your fleet who have the proper permissions. Fleet banks are universal; items and Energy Credits placed into any fleet bank will appear in all fleet banks throughout the galaxy and are available for retrieval no matter which fleet bank is visited.
Fleet banks are a part of the normal bank terminal and can have up to 8 tabs of storage space. The first tab is available to the fleet automatically; additional tabs must be purchased for a cost of 1,233,000 each.
Fleet emblem and uniform
In Star Trek Online, fleets can also create their own specific emblems and uniforms. These can be previewed and edited (if a member has sufficient permission) in the "Uniform" section of the main fleet window. Members can then visit a tailor and activate their "Fleet uniform" slot. The uniform may then be chosen as a pre-made costume option, whereas the emblem can be displayed as a shoulder patch on a member's character and as a painting on his or her ship.
There are a few simple requirements to be met to create a fleet (pre-Season 6 update):
- You must have 5 players to form a fleet.
- The 5 players must be joined together in a team.
- All the team members must either be Gold members or Silver members that have fulfilled certain requirements (see below).
- Visit the Fleet Ambassador (for Federation fleets, the Fleet Ambassador can be found at Earth Spacedock down the steps to the right of Admiral Quinns office on the Operations Side ( go around the wall where the waterfall is as you come out of the transporter), and for Klingon fleets the Fleet Ambassador can be found in First City on Qo'Nos just to the left as you enter the Barracks) to create your fleet.
A number of people have posted on the forums about the exact make-up of the team members required to create a fleet. Here are some of the things that they have found:
- The fleet founder and forming members don't have to be Gold members. A Silver member that meets any of the following requirements can also be a fleet founder or forming member: 20 hours of aggregate play across all characters, account has ever had a subscription, account carries a ZEN balance, account has made a C-Store purchase. 
Fleet Size Limitations
While a team of five is required to form a fleet, once formed it can go down to a single member. The maximum limit is 500 members, after which invitations would give the error "[Fleet] Error inviting to fleet: <Fleet Name> has reached full capacity. You must remove members to add new ones."
Some other tidbits about fleet creation:
- You can get around the 20 hour trial period on an account by getting enough dilithium to trade in for at least 1 - that will give you a ZEN balance without having to purchase ZEN. 
- Once a Silver member gets any ZEN (by any means), they need to log off and log back in in order to qualify for creating a fleet.
- Once the fleet is created, you can drop below the 5 member requirement.
- All players in the Team must be present on the same map instance as the Team Leader.
- Star Trek Online Forums - Fleet Creation - Post#11
- Star Trek Online Forums - newly created accounts can't receive fleet invites?
- F2P Features Matrix - Star Trek Online Official Site
- Star Trek Online Forums - newly created accounts can't receive fleet invites? - Post#21
- Fleet actions, a form of group-instances which, despite their name, do not require the participants to be in a fleet.