An android is usually presented as a purely mechanical being with levels of strength and mental agility far exceeding their human counterparts. They have all the advantages of human dexterity, yet also have an infallible memory and incredible intelligence.
Due to current technological restraints, the only place where androids can be found is within the realm of science fiction literature and films. Although a lot of the separate parts of an android have been successfully built, no-one has been able to combine them into a single structure. For example, synthetic skin is very much a reality and is often used by puppeteers... Japanese scientists have created a set of legs with enough stability to climb significant inclines and even steps. Unfortunately they weigh approximately 35kg.
The greatest hurdle in creating an android is definitely making the brain. Today's cognitive scientists have created a computer program which will ask questions, but this is still not proof that it is self aware. The 'computer psychiatrist', Eliza, was able to ask questions but she certainly didn't know how to think for herself. Learning programs have been created using neural networks which incorporate a model of a neuron, the building block of the human brain. It is thought that the complex interactions of literally billions of these neurons are what gives rise to intelligence.
The science fiction ideal of what an android should look and act like are far more advanced than today's technology permits. Image-wise they come in two flavours: the metallic 'robot', such as C-3P0 from Star Wars, and the synthetic human nearly indistinguishable from the real thing, such as Data from Star Trek: The Next Generation. Where personality is concerned, nearly every visionary sees the same thing: androids all respond, act and are usually perceived as being individual entities rather than merely a tool owned by someone.
Marvin the Paranoid Android, a key character in Douglas Adams' Hitchhiker's Guide to the Galaxy, is a prime example of an android. He professes to know absolutely everything, is practically immortal due to the design of his body, and can accomplish almost everything from the most advanced calculation to waiting around for a billion years doing nothing in particular.
Other science fiction androids include Bishop from Aliens, Kryten from Red Dwarf1, and Rachel Tyrell from Blade Runner who is a gynoid, the female equivalent of an android.
Cyborgs are a different type of being, often confused with androids. Cyborgs are biological entities which have been modified, usually to a great extent, by mechanical means. Famous cyborgs include Robocop and the Borg Collective featured in Star Trek. Either very little is left of the body, such as in Robocop, or it is used as a housing for mechanical enhancements which can be used to replace dysfunctional body parts, such as a synthetic hip joint, or even replacement ears and eyes.
Scientists point out that, at a fundamental level, many humans are close to being cyborgs at some point, using mechanical devices which allow us to perform otherwise impossible tasks. Examples include riding bicycles and using wheelchairs.