Hexes are types of spells which are generally used for a negative purpose. It is performed like any other black magic spell but here there is usually a specific and often temporary purpose the spell has to full fill.
It is like sending a guided missile with a specific purpose. For example a black magic user can cast a spell upon someone so that they has bad luck in their exams. Now this spell will find out that person and attach itself to his energy or aura. It will remain there dormant until the person enters the exam and during the exam it will activate. Now it will alter the framework of the persons luck (read more on luck’s framework here) in such a way that chance favors him very less, to the point that he may even fail the exam !
Hexes are similar to Jinxes and curses but the former is rather weak and the later is usually very strong and long lasting.
If you were to compare them :
Black Magic Curses > Black Magic Hexes > Jinxes
The word ‘Hexes’ is derived from Hex which means ‘six’. Often in Black magic six sided shapes (Hexagons) are used as the basic framework for a seal. And it is through these seals most of the magic is enacted. Furthermore 666 or three sixes (or Hex-es) is the numerical value associated with the Devil, Lucifer or Satan.