It is a sturdy, long, thin, straight rod typically made of metal, rigid plastic or carbon fiber, and usually has a handle at one end for gripping and threadings at the other end for attaching cleaning accessories.
It is used to push brushes, mops and cotton patches through the bore to deliver solvents and scrub away foulings, or to physically remove any obstructing objects such as a squib load.
Commonly, a breech-end extension tube, known as a bore guide or rod guide, is also used in conjunction with the cleaning rod to prevent cleaning solvent from spilling out into the receiver, and to keep the rod supported and centered inside the bore in order to minimize potential damage to riflings.
A cleaning cord often has one or more integrated brushes to help clear away more stubborn contaminants, and may also be used to apply lubricants.
The screw thread sizes and "genders" used for attaching brushes to the rods vary between different calibers and manufacturers.