The word Sefirah is close to sappir, sapphire, and to sefar, number. The Sefirot are the ten degrees through which the light of Ein Sof descends along the kav and takes form. Sefer Yetzirah, the oldest text of this tradition, names them "ten Sefirot belimah," ten Sefirot "of nothing" or "held back," and hammers: "ten and not nine, ten and not eleven." The Pardes Rimonim opens on this point: all who engage in this wisdom agree that they are ten, "without any dispute."
Here are their names, from top to bottom. Keter, the crown. Chokhmah, wisdom. Binah, understanding. Then Da'at, knowledge: in the system of the Ari, it is counted in place of Keter when the latter is considered too high; Cordovero, for his part, counts Keter and sees in Da'at the hidden aspect of Tiferet. Chesed, loving-kindness. Gevurah, strength. Tiferet, beauty. Netzach, victory or eternity. Hod, splendor. Yesod, foundation. Malkhut, kingship. David's verse in Chronicles names five in a row: "Yours, L-rd, are the greatness, the might, the beauty, the victory and the splendor," then "for all that is in heaven and on earth," which the Zohar reads as Yesod, and "Yours is the kingship," Malkhut.
Etz Chaim reports three ways of arranging them, all true at their own level: as a ladder, one below the other; as concentric circles; or in three columns. It is this last figure that is called the tree. On the right, the column of kindness: Chokhmah, Chesed, Netzach. On the left, the column of severity: Binah, Gevurah, Hod. In the center, the column of balance: Keter, Tiferet, Yesod, Malkhut, with Da'at between Chokhmah and Binah. The Pardes devotes a whole gate to the "arbiters," the middle Sefirot that decide between right and left: for the world needs at times severity, at times kindness, but never without measure, and the middle line decides between them.
The tree is also read in triads. Chokhmah, Binah, Da'at form the intellect. Chesed, Gevurah, Tiferet form the heart and the qualities. Netzach, Hod, Yesod form action and transmission. Malkhut, at the bottom, receives everything and gives it back to the world.
The most famous description is Elijah's in the Tikkunei Zohar. He first says what the Sefirot are: "ten tikkunim," ten arrangements, "through which You conduct hidden worlds and revealed worlds," and through which "You veil Yourself from human beings." Then he warns: "whoever separates one from another among these ten, it is as if he separated within You." Finally he gives their order: "one long, one short, one middling," which the commentators read as the three columns; and their image in the body: Chesed the right arm, Gevurah the left arm, Tiferet the trunk, Netzach and Hod the two legs, Yesod the sign of the covenant, Malkhut the mouth, Chokhmah the brain, Binah the heart, Keter the crown on the head.
Keep Elijah's warning for everything that follows: the Sefirot are not ten deities nor ten independent forces. They are the instruments of a single Craftsman, and study consists in seeing how they hold together.
