She-Ra and her rebel friends must destroy the evil Hordak! - This is the ongoing plot of the show. She-Ra is He-Man's twin sister. In many ways the show is a copy of the He-Man series, except that most of it's lead characters were female. Think of Hercules the Legendary Journeys and Xena: The Warrior Princess.
Most adventures take place on She-Ra's home planet, Etheria. On rare occasions, He-Man would visit from his home planet, Eternia. She-Ra's arch enemy is the evil Hordak, a powerful gangster / despot who usually plotted to take over the planet. Just as He-Man had a relationship with She-Ra, Hordak also had a relationship of sorts with Skeletor. The only thing different is that Hordak hates he-Man, She-Ra AND Skeletor.
He-Man would visit She-Ra by way of a dimensional portal or transporter-like system. Eternia and Etheria are part of the same universe, but it is unknown just how far apart they are.
Although He-Man was STRONGER, She-Ra also had more powers than He-Man. She could change her sword to just about anything she wanted to, and she had the power to heal as she did with Swift Wind.

And, just as Adam does with He-Man, Adora keeps "She-Ra" a secret and doesn't tell anyone.