Darkseekers (also called Hemocytes) are infected humans. The origin of this infection was a supposed cure for cancer.
Following the failed containment of the intial outbreak on Manhattan Island, the infection spread around America.
The dominating clan of Darkseekers on the island were eventually defeated by Dr. Robert Neville in the film following their assault on his home. Facing unwinable odds he hides the cure with two survivors as they take cover within a furnace and sacrifices himself by detonating a grenade killing him and the remaining Darkseekers.
There is also an alternative ending of the movie in which the alpha male of the Darkseekers wanted the specimen that Dr. Robert Neville had. He started smashing the glass door, paused, and made a butterfly with what presumably was mud. The specimen had a butterfly tattoo on her body. Dr. Robert Neville opened the door, unhooked the medicine and gave the alpha male the specimen then it walked out with the rest of the Darkseekers. This adds more depth to the film and allows us to presume that the Darkseekers are more human than we expected.
The major weakness of the Darkseekers was the unique side effect of their viral mutation, intolerance of ultraviolet light. When exposed to minute amounts to UV light their skin is rapidly eaten away and when exposed to full on UV radation of the sun they are killed within moments.