Early in the 21st century, a race of aliens, the Taelons, arrive on Earth. In exchange for being allowed to take refuge on our planet, the Taelons offer the people of Earth access to their advanced technology. As a result, disease, war, and pollution are nearly eliminated. Despite all these advances, there are some people who think the Taelons are not as benevolent as they seem. A resistance movement is organized. Ultimately, it is discovered that the Taelon aliens are not purely evil, nor are they actually trying to "conquer" the world (except Zo'or, the main antagonist in seasons 2-4). They can no longer reproduce — Da'an was the last Taelon to have a child — and are on the verge of extinction. Their initial goal is to see if they can use humanity to extend their lives. However, they do have a hidden agenda: to bioengineer the human race to help fight their mortal enemy, the Jaridians (who come from Taelon origin). In doing so, they have no problem violating human civil liberties to reach their goals.

Source: Wikipedia.org