The mask is a metal mesh which covers the face and a fabric 'bib' which protects the neck. Surprisingly, it is quite easy to see through the mesh but it can get quite hot in there. You must wear a mask everytime you fence, even if you are just practicing with another person.
When selecting a mask you should make sure it fits your head snuggly and has a strap at the back, this will prevent the mask falling off during a bout, this does happen but only as a result of poor fitting or a lack of maintenance. If you wear glasses, finding a mask that fits over them can be tricky but keep trying, they are available.
The mesh can be any colour or even have some design on such as your nation's flag, black is the most common though.