The mask is one of the most important pieces of safety equipment, protecting the face, head, and neck. At no point will any fencer participate in any activities where there is blade on blade engagement without a mask on. The mask is made up of a steel mesh face shield that covers the face and a heavy fabric bib that covers the lower head and the neck to the sternum on the chest. It is important that it overlaps the jacket. Also, the bib must be permanently attached to the body of the mask. Years ago, there were masks that had snap out bibs (so they could be washed or replaced), but those are no longer legal for competition fencing for safety reasons and, while not expressly forbidden for practice, are highly suspect and should be considered as less than safe for use.
Those are the most important components of the mask. Additionally, there are weapon specific masks that are also available for fencers who are participating in electrically scored competitions. Those special weapon masks are not necessary for dry (practice) fencing. A basic competition mask is sufficient for all practice in all three weapons.
The basic competition masks are usable and appropriate for fencing epee competitively. For epee, the mask needs to meet the above requirements for composition and coverage. The metal mesh will be coated with a non-conductive material. The bib will be non-conductive fabric.
Foil now includes a portion of the mask as valid target area at the bottom portion of the mask. As such foil masks feature a strip of conductive lame material along the bottom of the bib. Competition masks come with that strip built into the mask.
For fencers who have older masks from before the addition of the conductive strip, or those who've purchased a practice mask without the strip and have later decided to start competing and need to have the lame piece can purchase a lame patch that is sewn onto the bib to meet the requirement for tournaments.
Sabre has masks that are the most different from the other weapons, as the entire head is valid target area and so the whole mask needs to be conductive metal. Sabre masks will almost always be a bright silver, as they are not painted. Occasionally, they will also feature an acrylic "window" in front of the eyes, instead of the metal mesh. The sabre masks are only appropriate if you are planning on participating in sabre competitions and generally need to be in addition to a mask that is used for foil and epee, unless you are specifically fencing sabre only.
Mask Connector Wires -
Because sabre and foil both have conductive masks, in competition, they need to be connected to the body lame (and through that, to the body cord and back to the scoring box). The foil and sabre masks will have a tab on them that the wire connects to with a spring loaded clamp and then the wire will be clamped onto the collar of the conductive lame that you wear in those competitive weapons. The same wire is used for both of those weapons, so if you are competing in both foil and sabre, it is not necessary to purchase two sets.
There are no mask connector wires in epee.