Rebel forces are a group of individuals who engage in armed conflict against a state or other established polity. They use this conflict to seek political change. They may aim to capture the central government (center aims) or they may want to retain autonomy in their region (peripheral aims).
There are many reasons why rebels form, and they differ from one case to another. In some cases, a small number of wealthy families will fund and arm a band of terrorists. This band then goes on to commit terrorist acts against a state. Other times, a single leader can inspire the formation of a large group of rebels. For example, the Rebellion of the New Republic grew out of the clandestine meetings of Senators like Mon Mothma and Bail Organa in the final days of the previous republic on Coruscant. They supported efforts to shame Supreme Chancellor Palpatine into relinquishing his emergency powers.
Rebels are a part of the continuum of civil wars that have occurred throughout human history. They range from guerilla-style bands with few resources to well-funded, professional military units. They can be backed by either domestic or foreign backers. For instance, the United States covertly trained and supplied rebels in Syria for several years while France and the Kingdom of Saudi Arabia also aided them.
Often, rebel groups are designated by their governments as ‘foreign terrorist organizations’ or FTOs. This designation has a profound effect on the amount of material support they can receive. It may even stop the flow of arms altogether. Moreover, if rebels are captured by the authorities of their host country, they can be prosecuted for a broad range of crimes including treason, murder and possession of illegal weapons.