The Structure and Variability of Sagittarius A:Zooming into the Supermassive Black Hole at the Galactic Center

The Galactic center provides a unique astrophysical laboratory for us to study various astrophysical processes.In this paper, we review and outline the latest results from observations of Sgr A in terms of source structure and variations in flux density.Sgr A phenomenon represents a typical case of low radiative efficiency accretion flow surrounding a supermassive black hole in low luminosity AGNs. Many pending astrophysical problems found from observations of Sgr A have challenged the existing astrophysical theories.Current theoretical models of Sgr A are also reviewed.