Transformers are races of what was originally giant sentient robots with the ability to disguise themselves into things which fit in with things on planets they are on.
Transformers can almost always transform. They can transform into nearly anything given it is about the right size compared to themselves except on special occasions. There have been some known to transform into things as big as planets (ie. the Death Star Transformer) to things as small as insects. Regularly they are able to turn into types of vehicles because vehicles are closer to their size range as they usually fit in perfectly in regular suburban places locations (people see them all the time).
Size channging has been quite common in transformers over the years. Although scale hasn't been a very big concern, some transformers have been shown to quite clearly and legitimatly change their mass completely. This can mean either that a transformer the size of a regular transformer may turn into something the same size as something which can be used by other transformers such a guns or even small enough for humans to use, or that they can turn from the same size as other transformers to something a transformer can stand up and move about inside of.
Though a transformers origin may vary from franchise to franchise, their origin in real life is that around 1985 Hasbro invented a comic book line, a cartoon and a toyline of giant robots that transform and said franchises have adapted over the years. In fiction they may either be built by other lifeforms or have evolved more akin to organisms, however they all started on the planet Cybertron.
Generation 1, the first line of transformers, depicts the Autobots and decepticons as being built by the Quintessons, revolting and starting a war between two factions (autobots and decepticons) which then came to earth.
In another continuity of the G1 continuity family they are a race of robot aliens who mostly got along but when an evil robot known as Megatron was built he started a faction of evil robots known as decepticons who started the war.
The transformers timeline was the same as th g1 except that it was over 300 years into the future where they had instead scanned DNA of animals and would then turn into them instead of technology
In the live action movies the transformers were brought to life by the All spark which brought to life all Cybertron's inhabitants with 7 primes who built and evolved into a race until one of them turned evil and started the faction of the decepticons. These ones start their individual lives as protoforms but to stay alive they must be given energon to stay alive.
There are also many transformers who started out as regular peices of technology and were granted life by the All spark, for some reason being brought to life means they are given the ability to transform.
In Animated their origins are similar to the origin of the transformers in the movie. It is unknown how the two factions were created.
Most transformers have become able to transform into things they have scanned. They may scan things with either large scanning devices inside their own ships, smaller drones which are able to scan technology or have scanning devices inside themselves. Once scanned the transformer seems to be able to change parts of themselves into parts of the technology they scan. If not scanned a transformers alternate mode is usually what they were built as and they later learned to transform.
Transformers come in all shapes and sizes they are usually the same height as either a small house or garage but they can be anything from bug size to the size of a whole planet.
- Insect size: The insecticons from the movie "Revenge Of The Fallen" could turn into small insect like robots and the microcons could turn into what seems to be some form of mechanical ballbairings.
- Small animals: Many robots which were brought to life by the All spark in the first live action film known as "Real Gear Robots" could turn from small pieces of technology such as an Xbox, a phone or a steering wheel into living robots around the sizes of rodents through smallish mammals. Also the decepticon mini-cassettes could turn into robotic birds, bats or cats. In the beast era there was Rattrap who was supposed to be the size of a rat but was more like a capybara (big rodent) because he was the same size as a cheetah.
- Humans: The original decepticon mini-cassettes which included Frenzy and Rumble could turn into human size robots. the Frenzy from the movie could also turn into a human sized robot. There were pretenders which are basically the regular robots that are normal size that could completely disguise themselves as humans, Headmasters are humans that own a suit which can control a body of a regular size transformers body. A human girl called Sari in Transformers Animated was shown to be part robot as well so she was what could be considered a transformer. The mini-cons in Armada couldn't talk much but they could turn into tiny versions of vehicles and in robot mode they were around human size. The combined form of the above mentioned Microcons is a mantis like robot which is about human sized.
- Medium sized animals: In the Beast era many robots turned from things like big cats to robots which were slightly smaller than average.
- Regular sized transformers: Normal transformers that turn into cars, trucks, planes and other large vehicles can be from the size of a room in a house to the sizes of buildings themselves.
- Large animals: In the beast era there were some transformers that turned into things like dinosaurs which were a little bigger than regular transformers
- Large buildings: The original cartoon featured city bots but due to both scale issues and the fact that a city size robot would be hard to see next to normal transformers, they are only the sizes of big buildings. One character called Astrotrain could be around the size of a large building (or even the size of a building which is large to normal transformers) while in his train/plane mode which transformers ride inside of.
- Cities: The city bots were meant to be about the sizes of things like that, however that would be too hard to show them next to regular size transformers.
- Planets/moons: There were no transformers the size of countries or continents because those would be just plain hard to put in scale with about anything else but there are about 4 transformers that turn into planets or moons: Unicron, who turns into a planet with mandibles which eats other smaller planets (no, he isn't galactus). Primus, the creator god to transformers he at times takes on the body of the transformers homeworld: Cybertron itself. The Death Star Transformer which is a giant robot version of Darth Vader and the Death Star (a moon size spaceship) and Dead End, A minion/mini-con of Unicron who turns into a small moon for Unicron's planet mode.