Alliances allow you to work together with other players.
Mainly for smaller empires to be able to take on a larger one. But there are other scenarios where a player would chose an (temporary) alliance.
Anyone can create an alliance, but to join one you need to have met at least one of its members.
Members automatically share planet ownership data with other members of the alliance. Which helps keep everybodys map up to date. (see https://wiki.ferion.com/fog-of-war)
Information collected by any of the members on an enemy empire is automatically shared with all the members. (Planet/Ship count and current technology)
When creating an alliance, a group chat is automatically created with all the alliance members, allowing you to plan/chat ingame.
The alliance can work together to create prototypes of each technology. Which works as follows:
Only the alliance leader can select a prototype to work on.
And its important to note that the "discount" you get on the technology varies. The bigger the empire, the smaller the "discount".
Finally, when 2 players in the alliance already have the tech, a prototype is given to all of the members automatically.