fitted is a generic function which extracts fitted values from
  objects returned by modeling functions.  fitted.values is an
  alias for it.
All object classes which are returned by model fitting functions
  should provide a fitted method.  (Note that the generic is
  fitted and not fitted.values.)
Methods can make use of napredict methods to compensate
  for the omission of missing values.  The default and nls
  methods do.
fitted(object, …)
fitted.values(object, …)an object for which the extraction of model fitted values is meaningful.
other arguments.
Fitted values extracted from the object object.
Chambers, J. M. and Hastie, T. J. (1992) Statistical Models in S. Wadsworth & Brooks/Cole.
coefficients, glm, lm,
  residuals.