Life as recorded in the Bible
Cain (Hebrew, "acquisition") was the firstborn son of Adam and Eve as recorded in the Bible. Born after the fall, he became a farmer. He was the first murderer, killing his brother Abel because God accepted Abel's offering, but not Cain's. God punished him by cursing him to be a wanderer who would always have to toil hard for little reward. When Cain protested that "whoever finds me will kill me," God put a mark on him so that "if anyone kills Cain, he will suffer vengeance seven times over." 
"Who was Cain's wife?"
When Cain was sent away by God into exile, mankind was in its second generation - anyone Cain (or any of his siblings) could marry would be his close relative. However, the Law of Moses forbids the marriage of close relatives. Many people find this a Biblical contradiction.
However, this law was not given until later, at the time of Moses (Leviticus 18). Indeed, Abraham married his half-sister Sarah (Genesis 20:12). There is some logic go this in genetics: when God created man, He called him "very good". However, after the Fall of Man, harmful genetic mutations would gradually build up and cause virtually all offspring of close relatives to be deformed because both parents would have the same mutated alleles. For example, many Egyptian pharaohs married their sisters and had perfectly normal offspring; the children of such incestuous unions today are almost always deformed.
Note also that a wife is related to her husband even before they marry, as all humans come from the offspring of Adam and Eve.
Therefore, the popular question of "who was Cain's wife?" is easily answered.