They're not really a Celestial, so to speak. In fact, the name 'Cylestial' was given to them by researchers on the planet known as 'Earth' in their year of, I believe, 1980 as their actual name is unpronounceable in any tongue, even those of Celestials and Demonic origins.

To describe them would also be a bit of a task as each one is vastly different from the other. But, one common trait they share is a power called, in our tongue, 'Conjoinment', where they would become one with a mortal (Per the mortal's permission, of course) and said mortal would gain immortality, along with any power that the Cylestial possess.

There's only two that never conjoined with someone: Bastet and Taweret. While Bastet's reason is unknown to this day, Taweret's reason is simple: She rather help families take care of their children, which is why she has the nickname of 'Aunty Taw'.