The Sith Empire was an organization of Sith Lords and non-Force sensitive soldiers. It was all controlled by the Dark Sith Emperor. The Sith Empire planned an invasion of the Galactic Republic for centuries and took that long to prepare for the invasion. They took over the galaxy after many years of fighting, but shortly before the events of the Phantom Menace, the Catpire took the Sith troops by surprise and the whole Sith Empire collapsed.