  Recently, it was pointed out that quantum orders and the associated
projective symmetry groups can produce and protect massless gauge bosons and
massless fermions in local bosonic models. In this paper, we demonstrate that a
state with such kind of quantum orders can be viewed as a string-net condensed
state. The emerging gauge bosons and fermions in local bosonic models can be
regarded as a direct consequence of string-net condensation. The gauge bosons
are fluctuations of nets of large closed strings which are condensed in the
ground state. The ends of condensed open strings are the charged particles of
the corresponding gauge field. For certain types of strings, the ends of
strings can even be fermions. According to the string-net picture, fermions
always carry gauge charges. This suggests the existence of a new discrete gauge
field that couples to neutrinos and neutrons. We also discuss how chiral
symmetry that protects massless Dirac fermions can emerge from the projective
symmetry of quantum order.
