Kelly curated this post and obligingly sent it to me to repost, so thanks
Essentially, an android is a Human-like robot designed to mimic the appearance, mannerisms and body language of a flesh and blood person. However, even that definition is a flimsy one, as science fiction writers have come up with numerous weird and wonderful ways to create many new types of androids (at least on the page).
Sometimes an android is built by an alien race and thus resembles its creators. Sometimes an android is mostly robotic, but has some recognizably Human traits and sometimes an android is a bioorganic machine that is grown rather than built (making it very similar to the legendary homunculus creatures of ancient myth, or even the Golem of Jewish folklore).
The difference, essentially, between a robot and an android is that an android is designed to look more Human (for one reason or another) and a robot looks, well, like a robot (and thats opening a whole other can of worms, if youre interested in that, start with Al Jazari and go from there).
Why we want it?
The applications for Human-like robots are actually endless. Android firefighters could rescue people from areas inaccessible to Human beings, android surgeons would never miss a mark and their pleasant, Human-like appearance would certainly be preferable to some sort of scary surgical machine (if we were forced to choose, that is).
Androids could fill up many jobs that regular people simply dont want to do (but hopefully in a money-less environment to avoid putting people out of said jobs). If we are still using money by the time the androids show up, then android stock-brokers could use their computer-minds to analyse and prefigure oncoming trends months in advance, rendering the entire economy as one clear, safe equation.
Androids could actually be extremely beneficial to the world, but Im going to avoid the ethical consequences of their creation in this piece. You can discuss that amongst yourselves.
When can we expect it?
Robotics is an area of science that has come along in leaps and bounds recently. It may yet prove to be the breakthrough discipline of this era (following biology, psychology and, arguably, information technology before it).
In Japan, the Intelligent Robotics Lab have developed what they call an Actroid, which is an android by any other name. Able to mimic breathing, movement, blinking and speaking, these machines are actually quite a startling sight! However, despite running a complex form of A.I (artificial intelligence), the Actroids brain is actually accessed and operated via an external computer.
A bit more recently, in 2011, Danish professor Henrik Schrfe unveiled a robot version of himself. Hes not quite Noonien Soong, but its definitely a start!
I think well see realistic androids in at least some workplaces by 2050, but as to proper, artificial people, Im not sure that the majority of the Human race would ever actually allow such beings to be created. Having said that, if these early pioneering scientists get a consistently strong reaction to their creations, then I certainly wouldnt rule anything out.
Cool Factor (assuming they dont rise up and enslave us all): 5/5