There is a battle raging in the galaxy between various factions.
The galaxy is divided into sectors, and each sector is controlled by a particular faction. You can view the galaxy here.
If you are a member of a faction, you can choose to attack an adjacent sector or defend a sector that is under attack. You can only choose sectors that are adjacent to a sector that you already control.
If a faction attacks a sector, the players in the defending faction will be notified by email and have 3 days to select to defend it, otherwise it is automatically lost. Once a player chooses to defend the sector, this starts a game.
Your squad is replenished automatically. The maximum units you can have, and the calculation to work out that maximum, is shown at the bottom of your campaign squad page. You do not share units with your comrades in the campaign.