Cybermen: A "race" of cybernetically augmented humanoids. They vary greatly in design, with different factions throughout time and space. The two major groups are the Mondasian Cybermen, which are from the planet Mondas – Earth's twin planet in the Doctor's universe – and the Cybermen created by Cybus Industries, which originated on Earth in an alternate universe known by the Doctor as Pete's World. Despite the different origins, the cybermen shared many similarities. For the most part, they lacked individuality or names. Cybermen have no emotions and view them as a weakness. They frequently attempted to physically and mentally re-engineer humans and other humanoids into Cybermen, via a process called "cyber-conversion" or "upgrading". Beware the hands (they electrocute and shoot).
The Silence: A religious order created by Tasha Lem. They tried to kill the Doctor to prevent the fruition of a specific prophecy, which stated: "On the fields of Trenzalore, at the fall of the eleventh, when no living creature can speak falsely or fail to answer, a Question will be asked, a question that must never, ever be answered." The “question” is “Doctor who?”. If the doctor were to answer it, then the Time Lords would return and the Time War will begin anew. The Silence can summon lightning bolts, so be careful around them. Also, when you look away from a silence, you forget the encounter and the silence itself.