The four Knights of the Tarot.

Knight of Wands | Cups | Swords | Disks

Classical decks call these cards Kings. Qabalistically, these cards are the Yod in יהוה, corresponding to Atziluth or to the Sephirah Chokmah. In this way they bridge the gulf between Chokmah and Kether, which is in Atziluth. But that bridge is the eleventh path, Alef, which is the Fool, so in some sense the Knights are the Fool. Herein is wisdom ... or so I tell myself. Indeed do many things come to pass.

Read more about the court cards.

The following text is from Crowley's Book of Thoth.

The Knights represent the powers of the letter Yod in the Name.[1] They are the most sublime, original, active part of the Energy of the Element; for this reason they are represented on horseback and clad in complete armour. Their action is swift and violent, but transient. In the Element of Fire, for instance, the Knight corresponds to the Lightning flash; in the Element of Water, to Rain and Springs; in that of Air, to Wind; in that of Earth, to Mountains. It is very important as a mental exercise to work out for oneself these correspondences between the Symbol and the Natural Forces which they represent; and it is essential to practical Magical work to have assimilated this knowledge.