Electrical Wire Ferrules (or wire-end ferrules) are crimped over the end of a stripped wire to prevent fraying. They also provide a gas-tight connection that protects the exposed end of the wire from air and gases that lead to corrosion.

These are sometimes called sleeve ferrules and are color coded for wire size in the DIN 46228 standard, confusingly with French and German codes for the same cross-sectional area in square millimeters. They are used to terminate stranded wire entering a screw or spring terminal and are required by CE certification for some electrical equipment.
Cable Entry Sleeves

Most cord end ferrules have a plastic cable entry sleeve that acts as a lip or flare to collect the individual strands of the wire as they are inserted into the ferrule, which helps maintain close association between strands of a bare conductor. This feature makes cord end ferrules a useful tool for ensuring the integrity of stranded wires that may be subject to frequent stripping and insertion into terminals.
Dual Entry Insulated Cord End Terminals, Bootlace Ferrules & Twin ferrules

These cord end ferrules are manufactured from tin coated high conductivity copper with Polyamide 66 insulating sleeves and meet DIN standards for wire containment. They let two cables be crimped simultaneously inside one ferrule, providing neat end terminations to tri-rated cable and multi-stranded wires.

These insulated cord end ferrules allow easy insertion of the wire into the terminal block prior to fitting the screw type terminal blocks or connectors. They are ideal for panel applications and are available in various multiple pin lengths to suit a wide range of terminal blocks and connectors.