From EVEWiki
Motherships are similar to carriers but considerably larger and are the second largest spaceships in the game. Motherships offer all the same features as a carrier, but in considerably greater amounts (double the number of fighters for instance), and are completely immune to all forms of electronic warfare. Furthermore, motherships contain a ship and items hangar in which the owning corporation can store combat equipment. Motherships can fit cloning vats, allowing rebirth for allied pilots should they be killed in action. This saves the pilot a lengthy trip back from the nearest station cloning vat, and gets them back into combat.
In material costs, a mothership costs around 15 billion ISK to make. This cost is considerably higher if one has to invest in the BPO's necessary for construction. Construction of a mothership takes approximately 3-4 weeks. Motherships, along with Titans, are the only ships that cannot dock at a station and so they must be permanently in space. This means players must either sit in them until they are destroyed, or leave them in a protected area when not flying one.
Despite being intended for Alliance level warfare, one of the first Motherships built actually went to 'The Establishment', which are a small group of pirates. This has caused some of the major Alliance powers in the game to begin controlling the construction of these behemoths.
It is not known exactly how many Motherships are currently in the game. Band of Brothers own two Wyverns from both of the Caldari Alliance Tournaments. Dusk and Dawn, The Establishment, the Mercenary Coalition and Red Alliance also have one or more Motherships in their possesion.
