Transformation (or Transform if you failed English class) is the process by which a Transformer alters his form between modes.
The technical details of transformation, as well as the manner in which the art was introduced to the Transformers themselves, vary. When the issue is addressed at all, transformation is usually treated as an innovation from some point early in Cybertronian history rather than something that was literally always with them. The Transformers were, at the time of their genesis, simply Cybertronians -- mechanical life-forms -- and became "Transformers" only later. However, just as often in Transformers history the "origin" of transforming is simply not discussed.
Transformation is not a trait limited to Cybertronians. Some sentient alien races (such as Junkions exhibit the same ability. The Quintessons also incorporated transformation into some of their later creations including the Sharkticons, Allicons, and Overcharge drones. Some examples of transforming robotic aliens, however, can be traced to colonization in the distant past by Cybertronians. The planet Paradron, is an example of this.
Special transformation types
Some Transformers, specifically the Autobot Jumpstarters and the Decepticon Battlechargers, are specifically designed to transform much faster than ordinary Transformers, usually taking less than half of an Earth second to switch from robot to alt-mode or vice versa. How much this ability depends on physical versus mental capabilities is unknown. Triple Changers are also stated to have unusually fast transformations (roughly twice the normal speed).
Multiple alternate modes
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, and of course the special physical construction granting those forms, the balance of the matter seems to be one of training.
One Transformers such as Proteus uses a process similar to reformatting to more radically alter his forms while transforming. This ability to start with one robot and alternate form and transform easily to entirely different variations without extensive mechanical work comes from unique metals in his body, and does not apply to most Transformers.
The city Transformers Metroplex and Trypticon are both specifically stated to possess devices known as transformation cogs that somehow control their transformation processes, and are essential to allow them to transform at all. Such cogs are rarely mentioned elsewhere -- so it is not clear if all Transformers require them, or Metroplex and Trypticon are special cases, perhaps because of their size.
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 defeated, 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.
Transformers generally seem able to shift modes at will, although in cases where they have suffered severe physical injuries, transformation is sometimes a strain. In one case, and one case only, a Transformer indicated that staying in his altmode was too energy consumptive, and had to revert to his robot mode: Warpath, when trapped in midieval England and low on fuel.
This problem seems to almost never come up, even in cases where Transformers are very low on energy, so its significance is unknown. Perhaps walking is simply more energy efficient than moving his tank treads.
Transformation is often accompanied by a distinct noise -- an 8hz pulse, repeated five times, with each pulse lasting a roughly equal amount of time, whose pitch can be represented by the absolute value of a declining (for alt-mode to robot mode) or increasing (for robot mode to alt-mode) sine wave. As a rule of thumb, the larger the robot, the deeper the pitch of the sound, and the longer the duration of the pulses. There is no general consensus as to what onomatopoeia should be used to represent this sound.
At this time, neither the cause of the Noise nor its relationship to a Transformer's ability to transform is known. It may just be a side effect to the transformation process.