  As one of the most active blazars, Mrk421 is an excellent candidate for the
study of the physical processes within the jets of AGN. Here we report on the
extensive multi-wavelength observations of Mrk 421 over 4.5 years, from 2008
August to 2013 February. This source was simultaneously monitored by several
experiments at different wavelengths: ARGO-YBJ in TeV $\gamma$-rays,
$Fermi$-LAT in GeV $\gamma$-rays, $Swift$-BAT in hard X-rays, $RXTE$-ASM,
$MAXI$ and $Swift$-XRT in soft X-rays, $Swift$-UVOT in ultraviolet, and OVRO in
radio frequencies. In particular, thanks to the ARGO-YBJ and $Fermi$ data, the
whole energy range from 100 MeV to 10 TeV is covered without any gap.
  According to the observed light curves, ten states (including seven large
flares, two quiescent phases and one outburst) were selected. For the first
time, the multi-wavelength spectral evolutions of Mrk 421 during different
states were systematically analyzed. During the outburst phase and the seven
flaring episodes, the peak energy in X-rays is observed to increase from
sub-keV to few keV. The TeV $\gamma$-ray flux increases up to 0.9$-$7.2 times
the flux of the Crab Nebula. The behavior of GeV $\gamma$-rays is found to vary
depending on the flare, a feature that leads us to classify flares into three
groups according to the GeV flux variation. Finally, the observed radiation
spectra above 0.3 keV of different states can be reasonably described by a
simple one-zone synchrotron self-Compton model. The underlying physical
mechanisms responsible for different states may be related to the acceleration
process or to variations of the ambient medium.
