You can visit wikipedia.org for a full length definition of Cuckold.

A Cuckold is a man who’s wife cheats on him also known as an Adulteress. She plays a more dominant role in the relationship and is known today as a “hot wife”. Sometimes a man is her Cuckold because he can not tell her no or can not stand up to her. He will not leave her even though he knows she sleeps with other men.

Cuckold as a fetish is a little different because the man is aroused by the thought of his wife being with other men. This can include swinging, domination, feminzation and humiliation of the husband. When playing out the domination, feminization and humiliation part of the fantasy, that would be part of a dominant/submission role that is known in the BDSM community.

In many Cuckold fetish relationships a couple will seek out a “Bull” male or a “Stud” and the husband will just be a voyeur while his wife has sex. There may be nothing more to it than this.