Fleets are groupings of ships that move and attack together. This provides a huge advantage in combat, as the volley of fire will focus on eliminating an enemy ship quickly while providing protection to individual ships through a safety in numbers strategy. Unless facing an overwhelmingly more powerful opponent, enemy ships will usually focus on only one ship in your fleet, allowing the rest of the ships in the fleet to fire at the opponent unscathed.
To create a fleet, move the desired ships into the same sector. Select the ships you wish to add to the fleet and click the "Create Fleet" button. The max size of the fleet is determined by logistics. Smaller ships have a lower logistics cost. The total logistics cost and max is shown in the selection window, and you will not be allowed to create a fleet if the total exceeds your logistics. Double clicking on a fleet will open up the fleet manager, allowing you to add or remove ships from the fleet if needed.
You may mix different ships into a fleet. However, a fleet will only travel as fast as the slowest ship, so it is wise to attempt to keep all ships with the same propulsion capabilities else forfeit the extra movement points of the faster ships.
Fleet combat takes place in a special viewer, allowing the player to see a play-by-play sequence of the battle. Use this to analyze the effectiveness of weapons and armor and gauge if you need to redesign your ships.