Transformer
The Transformers are a species of robots (mostly) originating from the planet Cybertron. Though this is the most common term for them, there are others, mainly "Cybertron" as a reference to any being whose origina traces back to the planet in question (the adjective form being "Cybertronian").
The name stems from the race's generally-shared ability to "transform," to change from a robotic primary mode (usually, but not always, humanoid) into an alternate form, generally vehicles, weapons, machinery, or animals. In some continuities this abillity to transform is innate to all members of the species, in others it was a wartime innovation that was adopted by most, but not all, Cybertronians.
Biology
Transformers are living beings, despite their (usually) mechanical bodies. Many continuities portray this life as being granted by a soul-like spark. Traditionally, Transformers are fully-mechanical beings. But by the time of the "Beast Era", synthetic flesh has been incorporated into their construction (a technology that had been introduced earlier with Pretender technology, though in that instance the sythetic flesh was a removable outer "shell"). By Beast Machines, true organics had been infused into their biology, making them "technorganic" with an increased range of powers. The "Parasitech" techonolgy introduced in the Generation-1-based Kiss-Player series seems to have an organic component to it as well.
Transformers are generally large in comparison to Earth lifeforms, (two or three times the height of a human seems to be about average, though depictions vary wildly even among the same characters), but a Transformer can be much, much smaller or much, much larger. There are some small enough to fit in a human's palm (see Browning), while the Micromasters and Mini-Cons are two groups whose size range is within normal human standards in a time where most Transformers are larger, while human-sized appears to be the norm for Transformers altogether in the Beast Era. Others can be a good deal larger, like the colossal city-bots Metroplex or Fortress Maximus (the latter depending on which continuity he's in at the time). In the Cybertron series, the planet Gigantion is populated by both extremes: robots who tower over "normal"-sized Transformers, aided by their tiny Mini-Con partners.
While many Transformers retain a "humanoid" form with two arms, two legs and a head atop their torso, it is not uncommon at all for there to be extensive variations. Some Transformers do not have a "humanoid" mode at all, rather their "primary" form is that of an animal or vehicle. Generation 1's Ravage, Grand Slam and Raindance, for example, or the Beast Wars Mutants.
Some have unconventional limbs, their hands (or even more or their arm) replaced with tools, claws, weapons, or some other form of manipulator (like a beast-mode's head!). Some Transformers' lower bodies feature digitigrade legs, or wheels/treads instead of feet, or no discerable separate legs at all. Some even have more (or less!) than the normal number of limbs altogether. This design style became much more common in the Beast Era (Beast Wars Quickstrike and Beast Machines Thrust, for example), and is also common among the Mini-Cons, but Generation 1 is not without its examples (like Shockwave, Scorponok, and Octus).
Transformers' faces also run the gamut of design, from looking like a metal human in a helmet, to faces that can only be described as robotic (again, like Generation 1 Shockwave, whose face consists of little more than a single glowing eye) or animalistic (again, the Beast Era). The Mini-Cons are also notable for more commonly having non-humanoid faces.
Unlike humans who need air and water as well as fuel, the Transformer's only major biological need to assure their continued functioning is fuel (thoguh again, even this can vary depending on the fiction). Energon is the most common and prefered form of Transformer fuel, but other alternitives may be substituted if it is unavailable. By the Beast Era, many could ingest organic substances to supplement their fuel needs.
It is not uncommon at all for a single Transformer to change bodyform many times across their life-span. The ease of which they can do so depends on the continuity's technology level. In some, changing bodies requires an extensive full-body rebuild. Sometimes all it takes it to transplant their brain or personality program into a compatible new body. Other times, the body itself will be "reformatted" on the spot using a scan of a local compatible alt-mode.
For more information on Transformer biology see:
Culture
The most distinctive facet of Transformers society has been its near-continous state of war.
Toys
The Transformers were introduced by Hasbro in 1984 with the toyline know known as Generation 1 and have gone through numerous incarnations since, with the latest being Classics.
