A Base is a planet based civilian settlement or military outpost. They share some similarities with star ships, in that they can have 'hull' (structure), armour and weapons - but they cannot move. They are fixed in local on a planet, which makes them sitting targets to some extent.
There are two principle types:
Bases have STRUCTURE in place of HULL. It is normally divided into four rows. Civilian bases can be very large and have lots of structure.