Diplomacy

Diplomacy allows various treaties, declaring war and negotiating peace, alliances, etc. Establishing treaties generally requires a high enough level of relations with the other Empire which can be influence by things like previous treaties, Empire picks, events, etc.

Trade Treaty
are the easiest treaty to obtain and provide to both Empires after an initial few turns of negative income to establish trade routes, etc. Their effect may be enhanced with the and.

Mining Treaty
require a higher level of trust and allow Empires to establish to exploit resources within each other's. Their effect may be enhanced with.

It is generally best to make sure you're exploiting all the, , etc., in your own territory before entering into such an agreement--once you have done so, you lose nothing by entering into the agreement and open up your--friend's--resources to your.

Mining treaties allow empires to purchase strategic resources from each other for a certain cost per turn.

Research Treaty
require an even higher level of trust and provide to both Empires after an initial few turns of negative income to establish the various committees, lines of communication, etc. Their effect may be enhanced with the and.

Research treaties also allow trading technologies.

Alliance
Alliances require the highest level of trust.

Allies can see each other's discovered systems, colonies, fleets, leaders, technologies, etc., and settle planets within each other's borders.

They can coordinate their efforts by requesting each other to make peace or war, break treaties, research specific techs, or provide ships. Neither is required to accept these requests, however--one side can be at peace with an empire the other is at war with, for example.

All members of an alliance win if any member does, though this can be disabled in the settings.

Surrender Pact
Empires facing inevitable defeat can now offer or agree to sign a surrender pact thus removing the need to completely eliminate them from the game.

As subject empires, they provide their overlord a hefty percentage of their surplus credits, research points, culture, ship support and strategic resources each turn.

Subjects vote for their overlord and contribute to all victory types.