I’ve watched this show for years and been to a couple of the conventions, but even I don’t understand it fully.
Basic idea is - Earth and a lot of other habitable planets in the solar system are linked by a system of Stargates. Each Gate is made of a mineral called naquadah that has the ability to contain a stable wormhole. Consequently, by stepping through the wormhole you can travel halfway across the galaxy in a few seconds.
Each Gate has its own DHD - Dial-Home Device, which is basically a funky little control panel. You enter the seven-symbol address of the Gate you wanna go to, press the button and whoosh!
The people with the metallic things on their foreheads - they’re called Jaffa. The main bad guys in the show are called the Goa’uld, and they are actually little snake-type things that infest humans and take them as hosts. Having done so, they often attempt to use humans as slaves by impersonating their gods, so we have heard of Goa’ulds called Osiris, Ra, Apophis, Hathor, Heru’ur, etc…
The Goa’uld armies are the aforementioned Jaffa. Before a Goa’uld snake-thing can take an adult host, it takes part in a symbiotic relationship with a human - ie, the Jaffa. The Jaffa keep the snakes in little pouches of skin where their navels ought to be, and the snakes provide the Jaffa with an immune system.
Bad guys - well, only the Goa’uld. Apart from them there are these little metal bug things called the replicators, but I won’t go into all that just now. The Goa’uld are the main enemy and there are so many of them lurking all over the place that most of the time, they are the bad guys.
The good guys - well, because the Goa’uld took human slaves from Earth and sent them wherever they were needed, there are human slaves all over the galaxy (they all speak English, but that’s a story for another thread). These humans are generally good guys but not as technologically advanced as Earth is. Other good guys that are actual aliens are the Tok’ra (good Goa’uld - seriously!), and the Nox (pacifists, so they don’t get many storylines) and the Asgard (who look like the Roswell aliens). The Asgard are my favourite of the aliens for many reasons - they are much more powerful than the Goa’uld, and they’re on Earth’s side. Whoop.
As for human good guys, there’s the Stargate teams, the first one of which we follow the adventures of, but you probably know all about them.
It’s a good show, IMHO. It’s been getting worse as it goes on, though, which is a shame, but in its prime it was just damn good, with good storylines that tend to focus on people rather than the guns they carry, and one thing I’ve always liked is the fact the show never takes itself too seriously, and there’s a load of cleverly hidden jokes all over the place, which I won’t spoil.
Sorry for the sheer length of this…