The fleet panel is located in the top left corner (next to your order bar). By hovering your mouse on it it will automatically expand and display a list of the fleets / control groups you currently have under your control. You can create fleets by selecting a few ships and pressing [ctrl] + [0] to [9].

For each fleet it shows its location, the number of ships and their current orders. Double clicking one of the fleet will center the camera on their leader (you can take command of the leader with the dedicated hotkey).

If you are playing as an empire, you will see from time to time a red fleet called 'Auto: Invasion Fleet'. This is an automated fleet that is currently used by your empire to take over enemy sectors.