Classic Wiki Castanets

The castanets consist of two pairs of shallow, cup-shaped, pieces of special wood, usually chestnut (castana), although other woods and materials have been used in more contemporary times.  Each pair is drilled to receive an ornamental cord, which is looped round the thumb.  The pairs usually differ slightly in pitch; the lower is called macho (male) and the higher hembra (female).  The higher-sounding pair is usually held in the right hand.  The cups hang downwards and are manipulated by the fingers.

Source Carmen De Vicente

Castanets are a percussion Instrument. They are mostly used in folk or classical music especially in Spain. Castanets are played in pairs of varying pitches. It may either be attached to each thumb (folk) or to any or all the fingers (classical) depending on the type of music.

Source about.com

How to play castanets- the basics youtube video