The terms "sephira" and "sephiroth" are both related to Kabbalah/Qabalah, the mystical tradition of Judaism.
Sephirah is the singular form of the word, and it refers to each of the ten emanations of God. These emanations are often depicted as ten spheres upon the tree, with each sephirah representing a different aspect of God's nature.
Sephiroth is the plural form of the word, and it refers to the ten emanations collectively.
The sephiroth are often seen as a map of the spiritual journey, from the lowest level of material existence to the highest level of spiritual enlightenment.
In other words, "sephira" is the single emanation of God, while "sephiroth" refers to all ten emanations collectively.
NOTE: The use of the word "God" is a more traditional term used within a religious context, so the term isn't always used in the same way, but through the lens of the occult version of Qabalah, along with all its particular nuances, philosophical ideas, and interpretations.
