The Triads are noncorporeal conceptual beings born in the vacuum of space millions of years ago. They appeared separately, each with a different ideology (Order, Neutrality, Chaos) and reunited to achieve balance and completeness. They interact with other spacies through a large spaceship. Wehter the ship actually houses them or is only a device for interaction remains unknown.
Source: Babylon 5 universe. Babylon Wars Sourcebook.