Lore was an android built by Doctor Noonien Soong. He was essentially Data's evil twin (fortunately, the writers didn't give him the clichéd goatee). Unlike Data, Lore was able to show emotion from the time he was built. He claimed that he was considered "too perfect", forcing Dr. Soong to make Data's behavior less human-like. In reality, he had become dangerously egotistical and self-serving.
Lore was responsible for the destruction of Omicron Theta, the human colony where he was constructed. He allied himself with an alien called the crystalline entity, and it consumed all life on the planet. Lore later attempted to lead the crew of the Enterprise to the same fate. Exactly how or why he allied himself with the creature was never really explained, other than Lore mentioning a reward of some kind.
Lore attempted to organize and take command of a colony of Borg drones who had broken free from the collective and regained their individuality, but was stymied once again by the intrepid TNG crew. Lore was deactivated and disassembled in the aftermath of this incident.
Lore has essentially the same physical and mental attributes as Data, the only significant difference being his unstable emotions.