The Cybertronians, better known as "Transformers" are a race of mechanical lifeforms native to planet Cybertron.
Their bodies are made of a transformable metal called "Transformium", but the actual transformation power itself is enabled by a part known as the Transformation core, Transformation cog, or simply "T-Cog".
They are first manufactured inside their planet (which is also the alt-mode of their pantheon's chief deity Primus) and are granted a life force known as a "Spark", which comes from the Allspark. When they're first born, they have no alt-modes, and are referred to as "Protoforms". They gain their alternate forms by scanning a vehicle.
At one point they were believed to have evolved from naturally occurring pulleys, gears, pistons, and other mechanical parts. But this theory was proven faulty. Then a race known as the Quintessons claimed to have built Cybertron as a factory, and the Cybertronians were used for slave labor. But then rebelled against them. While this is true in some continuities, this is a lie in the storyline where Primus is their creator.
There are two main factions, the Autobots (the good guys) and the Decepticons (the bad guys).