Transformation

Transformation is the process by which a Transformer alters his form between modes. It should not be confused with reformatting a type of full-body upgrade.

Maintaining a transformed state requires a constant output of energy. The "root mode" for a Transformer seems to vary - sometimes robot, sometimes not.

Transformation is a mentally rigorous task, requiring the Transformer to focus on reconfiguring all the elements of his body in a way he or she isn't used to thinking about. By the time of the Beast Era (perhaps as part of the Great Upgrade) on-board computers had been added to Transformers' internal systems that coreographed the transformation process and reconfigured the Transformers' bodies for them. Post-reformatting, the new technorganic forms granted to Cybertron's inhabitants lacked on-board computers, requiring them to re-learn the mental disciplines involved.

Try turning on your back and walking spider-style like that kid in The Exorcist. Not easy, is it?

Special Cases

"Quick-Changers"

Some Transformers, specifically the Autobot Jumpstarters and the Decepticon Battle Chargers, are specifically designed to transform much faster than your average transformer, usually taking less than half of an Earth second to switch from robot to alt-mode or vice versa. How much of this abillity is physical and how much mental is unknown.

"Multi-Changers"

Robots with multiple transformations (Triple-Changers, et al.) are a special case. Though there seems to be some aspect of natural aptitude at play in one's ability to master multiple forms (Dreamwave comics referred to a "genetic potential" within the sparks of such robots), and of course the special physical construction granting those forms, the balance of the matter seems to be one of training. Most six-changers are said to have mastered their plethora of transformations through rigorous study.

"Adaptable Alt. Form"

Some groups of Transformers such as the Go-Bots use a process similar to reformatting to more radically alter their forms while transforming. This ability to start with one robot and alternate form tant transform easily to entirely different cariations without extensive mechanical work comes from unique metals in the Go-Bots' bodies, and does not apply to most Transformers.

The Action Master Skyfall appears to possess something akin to this abillity however.

Fiction

Marvel Comics

Unicron was the first Transformer, fashioning his metal prison into a robotic semblance of his original form. Primus, watching from a distance, was all like, "Oh snap, robots that turn into things? why didn't I think of that?"

Some accounts credit the development of transformation to Megatron and the Decepticons, with the Autobots later copying the technique. This, perhaps, implies that the potential for transformation instilled by Primus was dormant until Megatron realised it, or that it was forgotten and rediscovered at some point.

Animated continuity

Transformation was an Autobot innovation, developed during their first war with the Decepticons when it became apparent that they could not match their firepower and strength. The art of transformation allowed the Autobots to disguise their forms, thereby allowing for stealthy attacks on their enemies. This strategy met with great success, and the Decepticons were defeating, allowing the era of peacetime known as the Golden Age of Cybertron to settle in. The Decepticons subsequently adopted transformation technology themselves, coupled with robot-mode flight powers, and re-ignited the civil war.

The city Transformers Metroplex and Trypticon are both specifically stated to possess devices know as transforming cogs that control their transformation processes, and are essential to allow them to transform at all. However, no other Transformers are ever noted to have these doo-dads in them, so it's not clear if all Transformers require them, or if Metroplex and Trypticon are a special case, perhaps because of their size.

Transformation is not a trait limited to Cybertronians. The Quintessons later accquired the technology, presumably using it to create Sharkticons, while creatures as far afield as the lipoles of Io displayed the ability.

(Note: It was originally intended for the Lithones to be able to transform as well, but this idea was nixed. Whether or not this would indicate that the Lithones are, like the Junkions and Paradronians, former Cybertronians is unclear.)

Notes

From Teletraan I: The Transformers Wiki, a Wikia wiki.