The Galactic Senate was the governing body of the Galactic Republic. Representatives to the Senate were appointed by local governments, as when the Queen of Naboo appointed Padmé Amidala to be the Senator for Naboo. The criteria for having individual representation in the Senate is not clear; both the planet of Naboo and the interstellar shipping conglomerate, the Trade Federation, had representatives in the Senate.
The Senate's influence steadily declined following the election of Senator Palpatine to the position of Supreme Chancellor, until it had virtually no power when Palpatine declared himself Emperor. Palpatine finally dissolved the Senate entirely shortly before the Battle of Yavin.