Breather fabrics are normally a type of uncompressed felt, also called spun-bonded, needled felt. Various types and thicknesses are available depending on the temperature, pressure and resin type. The breather aids in removing air and volatiles from the laminate, and transports them across the surface to the vacuum port, as well as allowing the vacuum to be dispersed all over the bag area.
Bleeder (to bleed out excess resin) is fundamentally the same thing, but separated from the breather by a solid release film. It is most common to use 1 fabric to do both functions.