An android is described by most scientific outlets and professionals as any kind of robot designed to physically resemble a human being, both in appearance and behaviour. The concept of machines acting and looking like humans has been a very common idea for discussion in science fiction and scientific speculation, and the concept has been explored in many ways throughout history. This, of course, has meant that the idea has been developed and used by many authors of science fiction and fantasy writing, and it's frequently represented in many tabletop games and settings.
It is important to take into consideration that the original concept doesn't necessarily need the android to be an electronic-based creation; that idea appeared alongside the developement of computer science and AIs. Because of this, the idea of android is not limited to sci-fi settings, but it can also be included in fantasy settings.
Do not confuse with cyborgs. A cyborg is an amalgamation of a living being with artificial parts, whereas the android is fully artificial. Take that, whoever translated Dragon Ball!
Origins of the concept
The idea of machines looking and acting like us is not a recent topic of discussion. The first people to develop this idea were the ancient Greeks with their "αὐτόματον", complex artificial constructs designed to follow a series of instructions, usually resembling the human form. One of the most famous of them was Talos, a bronze automaton designed and created by the god Hephaestus (although in some versions the author is a mortal inventor) as a request by Zeus, and with the task of protecting Zeus's lover Europa. Talos's job consisted in guarding the island of Crete to avoid any potential threat to reach Europa.
Also one of the most famous takes of the android is the Jewish tale of the golem. A being magically animated created out of clay, it appears frequently in ancient Jewish folklore, although the most famous version of the myth is the Golem of Prague, where a rabbi created a golem to protect the Jewish community of the city, and had to dispose of the thing after that, usually after a series of violent events.
The first takes of the modern android, a machine based on science and technology instead of magic or divine powers, appeared during the 19th century in some speculative histories about "mechanical dolls". The term would not become popular until George Lucas brought the word "droid" into Star Wars. If a droid was just a robot, an android is a droid that looks like a human ("anthrop-" is the Greek root for "human being"; the term "gynoid" is derived from the same principle and used to describe droids that resemble women. Of course, the idea in modern media was older than that (see the movie Metropolis for a very early interpretation in cinema), but they usually just used the world robot, which is not as precise as it should be.
Actual developement of androids
Nowadays, the developement of human-like robots has risen in popularity, although the results usually fall right into the uncanny valley. Seriously, just look at some of those Japanese androids they show on those technology showcases, and tell me you don't get the creeps. The investigation also focus on the AIs, in particular those that allow self-learning. But as of now, they look and act very dumb. So your dream of getting your own robot girl is still too far. Don't lie, you want one.
Androids in Star Wars
The (trademarked) name "Droid" is obviously derived from "android". Despite this, while many droids in Star Wars have a roughly humanoid shape and artificial skin is old tech that has existed for a long time (a long time), very few droids could pass for human. The first known droid that pretended to be human did so merely by pretending to never meet in person and projecting a hologram of its "owner" and pretending they were elsewhere. That of course, doesn't really count.
It was not until the era of the Empire that droids capable of passing for human went anywhere. The first attempt came shortly after the fall of the Republic and was so disastrously deep 