Packer:

A piece of downhole equipment, consisting of a sealing device, a holding or setting device, and an inside passage for fluids, used to block the flow of fluids through the annular space between the tubing and the wall of the wellbore by sealing off the space between them.

It is usually made up in the tubing string some distance above the producing zone.

A packing element expands to prevent fluid flow except through the inside bore of the packer and into the tubing.

Packers are classified according to configuration, use, and method of setting and whether or not they are retrievable (that is, whether they can be removed when necessary, or whether they must be milled or drilled out and thus destroyed).