Circuits are an essential part of the anatomy of mechanical beings, especially Transformers. They regulate a diverse range of functions, not all of which are equally applicable in differing eras of Cybertronian history. However, they all share a common nemesis.
The function of bio-circuits is not explicitly stated, though by context it can be inferred that they are vital to a Transformer's continued functioning. After Starscream almost accidentally destroyed the Solar Needle, Megatron threatened to permanently deactivate his bio-circuits.
Brain circuits are vital circuits which maintain a Transformer's intelligence as well as a part of their personality. Grimlock once caused Optimus Prime to be late for a very important meeting with Tommy Kennedy (well...not really, Prime was just upset he was late), so the Autobot leader intended to have Grimlock's brain circuits upgraded. Good luck with that, Prime.
In the Marvel G1 comic, brain circuits were sometime referred to as cerebro-circuits.
Chameleon circuits are advanced circuitry that allows a Transformer to better disguise themselves from detection by others. Soundwave's various symbiots possess chameleon circuitry to better avoid detection by the Autobots. Ravage possesses special "umbra" chameleon circuits that allow him to blend into the shadows.
Apparently the part of a Transformer's brain that decides how to respond to a challenge. At the opening of the Beast Wars, when Rattrap was teasing Cheetor into a frenzy, Optimus Primal ordered them to "moderate [their] conflict circuits."
The electro circuits are a component within Transformer anatomy, but their purpose is unclear. Anti-electrons attack such circuits and cause severe glitches to occur in a Transformer.
Logic circuits are a component of Transformer anatomy that is charged with intelligence and reasoning out confusing situations. In the Beast Wars series, these circuits were especially important due to the increased reliance on beast modes. With the acquisition of beast modes, there came instances of the beast modes' instincts influencing the logic circuits. When Inferno came online, his beast mode convinced him that he was a real fire ant and that his stasis pod was his colony. Later, the Predacons stole the shielding system of the Axalon, forcing the Maximals to remain in beast mode to avoid Energon buildup, all while under bombardment and suffering from sleep deprivation. Eventually, their beast modes came to override their logic circuits. Only with Tigatron's help did they overcome their baser instincts.
Implying that another Transformer has no logic circuits is a common (mild) slur on their character.
Memory circuits are a component of Transformer anatomy that allows access to memories. They can be bypassed in order to make a Transformer "forget" certain things, such as who their enemies are.
Contaminated oil resulted in the Constructicons damaging their memory circuits, thus causing them to completely forget previous events.
A Transformer's oral circuits are related to vocalization. In the UK Marvel Transformers comic, Prowl cleared his oral circuits to cover up his daydreaming during one of Optimus Prime's boring lectures.
In the live-action movie, Bumblebee is described as having had his "vocal processor" damaged in a battle with the Decepticons, rendering him unable to communicate with other species, though still able to communicate over inter-Autobot frequencies. Ratchet had been having trouble fixing them ever since.
It is unknown how, if at all, the vocal processor and oral circuits are related. They may coexist, or they may be different devices which serve the same purpose.
Reason circuits are a component of Transformer anatomy that governs their capacity to reason. They are evidently distinct from logic circuits. In the Generation One cartoon, after being hit by a missile from Rumble, Red Alert became increasingly paranoid (far more so even than usual) due to damage made to both his logic and reason circuits.
A Transformer's security circuits prevent outside intrusion onto their computers.
Security circuits briefly remain offline after a protoform goes through its activation sequence. Megatron took advantage of this to change Quickstrike and Silverbolt's activation codes by remote override in order to make them believe they were Predacons.
Servo circuits are essential for a Transformer to remain standing. Without them, Transformers collapse as if paralyzed.
Among Rattrap's bag of tricks was a key-sized device which could disrupt another Transformer's servo circuits and disable transformation. The device worked only at extremely close range and appeared to be single-use, as Rattrap threw it away afterward.
Truth circuits measure and assess the accuracy (or possibly credulity) of spoken statements (kinda like Pinocchio's nose, but more high-tech).
Optimus Primal once refrained from offering Megatron the conversational pleasantry "glad to see you" rather than risk burning his own out.