The E-100 Series Robots were robots created by Dr. Eggman during the events of Sonic Adventure. They were created to help Dr. Eggman on his plan to conquer the world and reclaim the Eggman Empire. However one of the robots, Gamma, after discovering what he is forced to do, decides to stop Eggman's plan by sacrificing his brothers, Beta, Delta Epsilon and Zeta, and himself, which frees a small animal inside of each one of them.
Most of them were defeated by Gamma during the events of Sonic Adventure, but some are still alive. Example Chaos Gamma from Sonic Battle, E-123 Omega from Sonic Heroes, E-1000 from Sonic Adventure 2 and etc.
There also is some E-100 robots that exists on other Sonic media, such as the E-105/E-106 series from Archie's comic series, but those are non-canonical to the games chronology.