There are cyborgs among us. Most do not look at all like cyborgs - hulking abominations of flesh and machine wrought into existence by a cackling madman in a white lab coat - indeed, you yourself may be a cyborg without even knowing it (But don't worry, I don't mean in a creepy amnesiatic manchurian candidate sleeper cell kind of way). A Cyborg is simply a living being who's body has been purposefully augmented by some technological means, often with some part of the body replaced with an artificial component
There are of course different levels, or 'classes' of cyborg based on the amount and intention of the modification, and a quick explanation of each follows.
For this argument to make more sense, think of yourself as a character in an RPG [role playing game]. Or pretend that your resume has a 'stats sheet' listing your strength, speed, agility, intelligence etc. as some value from 1 to 20, with the absolute peak of human perfection being someone with 20's across the board.
Class 1: A Class 1, or restorative cyborg is a person who's physical or mental capabilities are for one reason or another less than the human norm and has thus been modified in order to raise said capabilities back to normal human levels.
Good examples of Class1's are people who are without the use (or in some cases the presence) of their legs, and instead have artificial legs of one type or another. Anyone with an artificial heart valve or indeed an artificial heart is a Class1. I myself am a Class1, as a team of doctors installed a permanent metal staple into the bottom of my jaw after it was broken a few years ago - the idea being that the staple would help restore some of the structural integrity that was lost when the bones were first broken. Not to mention that I have fillings in at least a few of my teeth (as I imagine you, dear reader, do as well). Anyone who wears glasses or contacts, or who has had laser eye surgery is a Class1.
[EDIT: disregard the comment on glasses and contacts, as they are non-perminant and non-internal, and if they are counted as Class1, so would "that nifty 'augmentation': clothes." - Nate Smars]
When you think Class 1 cyborgs, think of the modification as restoring something that was lost. (Someone who had 12 speed but broke their leg and went down to 9, then used some form of tech to bump themselves back up to 12.)
Class 2: Upon reaching the definition of the Class2 we move officially into the realm of the nifty, and in many people's minds into the realm of science fiction. A Class2 Cyborg is again someone who's abilities were sub par, but have since been modified in such a way that they are left 'ahead of the game' so to speak. One of the most well known fictional examples of a Class2 in popular culture would have to be Geordi La Forge from Star Trek: The Next Generation. His character (for those of you who somehow don't already know) was born blind. However with the use of that spiffyvisor of his he is able to see not only the usual EM spectrum that humans are sensitive to, but numerous other types of radiation as well as anything else the writers had need of.
For those of you more fond of pop culture fantasy rather than SciFi - think Mad-eye Moody from the Potter series. He replaced the normal eye he lost with a magical eyeball construct that could rotate and swivel more freely than a normal eye, as well as see through the back or sides of his own head (and probably any other matter if he really wanted). Mad-eye is therefore one of the few Class2 Cyborgs in the Harry Potter series.
A perfect example of a Class2 in real life is Oscar Pistorius, the so called 'fastest man on no legs.' Oscar is a double-amputee who's personal best of 46:25 in the 400 meter brought him juuuust short of the 45:55 qualifying time for the 2008 summer Olympics. But while there were certainly countless athletes who found themselves just that tiny bit short of qualifying, none of them but Oscar could ensure they made the qualifying run next time by calling a design firm and ordering a faster pair of legs.
In a nutshell, Class 2 cyborgs are still replacing something that was lost, but the replacement (either intentionally or not) ends up being better than what was being replaced. (instead of the tech restoring them to a 12, they end up at 14)
Class 3: And finally we reach the Class 3 cyborg, the 'true' cyborg. A class3 is someone who willingly modifies him or herself [or is modified on behalf of someone else's will] with the intent of becoming something more than they were before. There may be nothing wrong with a Class3 before they are modified, no physical or mental detriment that is being compensated for. Were I to replace my eyes with cybernetic implants just because I think it'd be cool as hell, I would be a Class3.
Back to Trek examples, the Borg are Class3s. While the individual may not be all too willing to be assimilated, the intention is still to improve on the existing biological system.
One real-life example of a Class3 is the UK's own Kevin Warwick. Warwick is not by any means one of the planet's most prominent Class3s, though. Any human female who has chosen to control her own reproductive cycle by modifying her own body's hormone balance[any chick on the pill] is a Class 3 Cyborg. She has significantly modified her own body for her own gain (ok, for her bumperbudy's gain as well).
Class 3 cyborgs are not making up for any prior loss or lack with their modifications, and are instead modifying themselves purely to become better than they were. (skipping the step of breaking their leg and going strait for the modifying tech to change their speed from 12 to 14 just so they can be faster.)
Class 4: The class 4 cyborg is not defined by intent or resulting abilities due to the modification the individual has undergone, but rather by end result and % of surviving original tissue. A Class4 is at least 51% artificial. Think Robocop here.
Once we pass the 51% mark the individual can be just a human brain in an artificial body (the most extreme 'upgrade' possible) and would still be a Class4. Only barely defined as human, Class4's are a long way off but still kinda spooky to think about.
So go now, and look with your newly educated eyes at this world filled with cyborgs. And be sure to call me as soon as you hear about anywhere that's looking into cyber-eye implants.