1. In Egyptian mythology, Seth is one of the principal deities.
"Set, also spelled as Seth, Setekh, Setesh, Suty or Sutekh. Generally speaking, in Egyptian Mythology, Seth is seen as the god of chaotic forces and violence. There has been considerable controversy over his actual role in ancient Egypt. The templates of priesthoods of Seth are mentioned in the text [of The Prism of Lyra] in reference to worship of the Dark Forces."
Seth was the (half-)brother of Isis and Osiris. After their father, Ra, divided his kingdom into Lower and Upper Egypt, and gave Seth rulership over Upper Egypt, and Osiris over Lower Egypt, Seth schemed to overthrow Osiris. Eventually Seth was able to drown Osiris, and before doing so, he castrated him. Isis, however, managed to raise Osiris from the Dead. Horus, son of Isis and Osiris, eventually became the ruler of a reunited Egypt.
Originally, in earliest times, Set was the patron deity of Lower (North) Egypt, and represented the fierce storms of the desert whom the Lower Egyptians sought to appease. However, when Upper Egypt conquered Lower Egypt and ushered in the First Dynasty, Set became known as the evil enemy of Horus (Upper Egypt's dynastic god).
Set was the brother of Osiris, Isis, and Nephthys, and husband of the latter; according to some versions of the myths he is also father of Anubis.
Set is best known for murdering his brother and attempting to kill his nephew Horus; Horus, however, managed to survive and grew up to avenge his father's death by establishing his rule over all Egypt and casting Set out into the lonely desert for all time.
In the 19th Dynasty there began a resurgence of respect for Set, and he was seen as a great god once more, the god who benevolently restrained the forces of the desert; but this was short-lived and by around Dynasty 20 or 21 Set became once more dreaded as the god of evil.
[Source: Shawn C. Knight, "Egyptian Mythology FAQ" ]
2. In the Bible, Seth or Sem, is the name of the third son - after Cain and Abel- of Adam and Eve. According to the book of Genesis, all men are descendants of Seth (or Sem) because the lineage of Cain was tainted, and eventually got extinct. The word 'semitic' refers to the name Sem. (But in that case, Sem was one of the sons of Noah, who had three sons of different races).