So, within the DS9 universe, there were the Prophets that lived within the wormhole.
What sort of beings were they? What were their powers?
The wormhole aliens basically had unlimited power inside the wormhole, existed mostly outside of time, and could spawn avatars.
The aliens appeared to have a vaguely defined amount of power outside the wormhole. The wormhole aliens could engage in Akira-style battles with their nemesis the Pah-wraiths. Who were of slightly less power, due to being kicked out of the wormhole.
The Deep Space Nine series does not go much further into their actual origins, but I know they are featured in the novels, of which I have read none.
The Star Trek Encyclopedia (considered a fully canonical guide to the Star Trek universe) explicitly refers to the entities living within the wormhole as alien beings. It does note, however that the Bajorans view them as spiritual beings rather than mere aliens, a major plot point in DS9 : In the Hands of the Prophets :
Prophets : In the Bajoran religion, the Prophets are spiritual entities who provide wisdom and guidance to the Bajoran people. Bajoran tradition holds that the Prophets were responsible for the nine Orbs that served as sources of wisdom for the people of Bajor. Many Bajorans believe that the alien beings first encountered in the Bajoran wormhole by Commands Benjamin Sisko in 2369 were in fact the Prophets These life-forms found the concept of linear time to be totally alien, and Sisko attempted to help them understand the importance of linear existence to Bajorans and humans. Ironically, although Sisko did not believe in the Bajoran religion...
As to their technology, it's not clear that their powers have any technological basis. Like the Q, Nagilum and various other energy beings, their abilities seem to be biological rather than technological. We can actually see this process occurring in TNG : Transfigurations where a physical being attains an energy state.
This is absolute speculation on my part but I believe the Prophets were once corporeal and may have evolved on Bajor. They say, many times throughout the series that they are "of Bajor" and have gone to great lengths to contact with the Bajorans and even protect them from an invading Dominion fleet at one point.