Setting

The story of Stargate SG-1 begins about a year after the events of the feature film, when SG-1 killed the supreme system lord Ra.
When the United States government learns that an ancient alien device called the Stargate can access a network of such devices on a multitude of planets they started sending specialized teams through the gate in order to explore the galaxy.
SG-1 is an elite United States Air Force special operations team, one of about 20 teams from Earth who explore the galaxy and defend against alien threats such as the Goa'uld, the Replicators, and the Ori. The series draws primarily upon Egyptian mythology, Norse mythology, and Arthurian legend.

Goa'uld: parasitic race. Wormlike creature connects to the victims brain taking over control.
         the Goa'uld are responsible for enslaving the people in the era of the egyptians posing as gods. Their         (stolen) improved technology made all this possible.

The Replicators: artificial race. consist out of nanites making them resilient and as the name says: they can replicate. they're known to attack ships in order to salvage the material to strengthen their number.

The Ori: A people turned (brainwashed) to believe in a certain philosophy by prophets.

The Asgard: the typical area 51 aliens. A friendly/neutral race with probably the most advanced technology available at the moment. they to had an influence on earth, leading to the worshipping of norse gods.

The Ancients: Extinct. As inventors of the stargate and other technology found in the galaxy, they we're the most advanced race in their time. We're researching the principle of Ascension.