A cufflink is a decorative fastener to fasten the two sides of the cuff on a dress shirt.
Cufflinks are designed only for use with link cuffs (also known as French Cuffs or double cuffs), which have buttonholes on both sides but no buttons. These may be either single or double-length ("French") cuffs, and may be worn either "kissing," with the ends pinched together, or "barrel-style," with one end overlapping the other. Kissing cuffs are usually preferred.
Cufflink designs vary widely.
The visible part of a cufflink is often monogrammed or decorated in some way.