The fyrd, in England before William the Conqueror took over, was the part-time military -- a bunch of amateurs who were duty-bound to serve up to two months per year in the armies of the king. (Anything past the two months was strictly voluntary.) The earls and thanes fought and also brought others from their lands to serve, but a large number of possible fighters would not be part of the fyrd unless fighting actually came near enough to threaten their village (in which case they wold run out and join their side.) Compare to the professional housecarls.