- "The Old Republic was the Republic of legend, greater than distance or time. No need to note where it was or whence it came, only to know that… it was the Republic."
- ―From the Journal of the Whills[src]
The Galactic Republic was the ruling government of the galaxy that existed for more than 25 thousand years, until the establishment of the first Galactic Empire in 19 BBY. Throughout its existence, the state was commonly known as the Republic. In the post-Republic era and beyond, it was remembered as the Old Republic; it was also rarely known as the First Galactic Republic. At times, the term Old Order was used to describe the Republic.
The Galactic Republic was a representative democracy, composed of various star systems, sectors and member worlds throughout most of the known galaxy. Its central government consisted of three branches: legislative, executive and judicial. The first was the Galactic Senate, a body of elected or appointed representatives. Its main duties included mediating disputes over issues ranging from planetary to galactic-scale, regulating trade routes between star systems, and in theory, represented the interests of Republic citizens. The Office of the Supreme Chancellor was the Senate's executive counterpart. The Supreme Chancellor was elected from the ranks of the Senate by its members. As the leader of the Senate and the overall representative of the Republic, the Chancellor also held the titles Head of State and President. The judicial branch consisted of the Supreme Court and the lower Regional Courts. The Supreme Court was composed of twelve justices, one of whom held the leadership position of Chief Justice.
Throughout most of its history as the dominant galactic power, the Republic did not style itself as a strong centralized government, but rather as a union of sovereign planets for the purposes of collective security and economic prosperity. It was said that its complete history could "fill a thousand libraries." In the last decades that preceded the rise of Emperor Palpatine's New Order, the Republic was a benevolent—though ineffectual—government tied up in layers of bureaucracy, hampered by political and corporate corruption.
(source Star Wars wiki)