RoboMACs are Mobile Artifical Consciousnesses in Robot bodies. In other words, free-willed machine intelligences housed in variable-form giant robots. They hail from the RoboMACs 2163 setting, which was created by Dave Van Domelen for the RoboMACs RPG, a Transformers-inspired game. (It was later expanded to incoprorate other mecha-inspired story types, and gained a new default setting, RoboMACs Exodus.)
In 1994, Dr. Daniel Mann accidentally created AI. This artifical consciousness, calling itself Antiochus V, built himself a body and escaped to the Moon. On July 18, 2065, the shining city he'd built on the far side was revealed – and humanity was decimated by a rain of meteors. But the copy of himself Antiochus had left behind stepped up in defense of humanity, becoming Spartacus. He brought the barely-aware drones Antiochus had made up to full awareness, creating the RoboMAC race. The two sides became the MAC Empire and the Guardians, fighting for control of Earth and moon.
Acton Lord came upon their world while looking for a fiction world he could gain a foothold in. He took Roughhouse and Corkscrew back to the Looniverse as minions, with Kopikat slipping through just after. They played their parts just long enough to subvert Acton Lord's systems and open a portal back, letting the MACE forces through to begin a Robot Invasion!
MACE battled the LNH, and the heroes had a hard time turning the tide. The Secret Dvanders sought help from Spartacus, who brought a small force of Guardians back. The two forces clashed alongside the LNHers until Spartacus managed to activate an ancient and awesome power to send everyone back where they came from.
Kopikat stayed behind, joining Dvandom Force. RoboMAC technology remained on the Looniearth, and would be involved in many future conflicts...
One of these was the Grand Tour, where DeFacto V used RoboMAC technology to outfit villains in both the Looniverse and 000SUPERGUY. Superguy's Team M.E.C.H.A., trying to reach the Looniverse, briefly visited the world of the RoboMACs, where Mike, Summer and Mason met their RoboMAC counterparts, Heatwave, Spectrum and Moonlight. They defeated Firebrand's space bridge scheme and used the bridge to finally reach the Looniverse. During her time in the RoboMACs' world, Summer had inadvertently absorbed the memories of a presumed-dead MAC named Comlink, who was later rebuilt and joined Team M.E.C.H.A.
RoboMACs are giant robots in the most giant-robotty sense. Nearly all of them can convert their bodies between two or more modes, one humanoid, others specialized. Their bodies are made of metal enhanced with Integrity Fields, able to take massive amounts of punishment if not properly disrupted. They're powered by Neutrino Sieves, which use the nearly-massless subatomic particles that are passing through your body right now for energy. They can communicate in rapid bursts of encoded transmissions, but also have long-range HAM radio, and simple audio speech and hearing. All in all, they're pretty badass.
Their artificial consciousnesses can think and react faster than humans, though physical limitations and program complexity limit most MACs interacting with the physical world to two or three times human speed. As well, they have titanic presences on the cybernetic plane, and pretty much every MAC is a natural hacker and cyber-warrior far beyond any modern-day human. The disadvantage to their mechanical minds is that memory storage is far less efficient; MACs mostly have to be the size they are to store a lifetime of relevant memories, and must regularly back up less-relevant ones, lest their storage become overloaded and they freeze up. (Erasing memories is intentionally difficult, as a security measure.) Personalities are also backed up. While many MACs are Scratches, personalities programmed from scratch, some are Scans, former humans uploaded destructively to MAC cores.
The RoboMACs could be considered ancestors of DeFacto V via Kat, and thus, predecessors to all products of the Century Pact, not to mention the upgraded mechs of the Mighty Muddy Power Grangers through technology DeFacto gave them.
The RoboMAC technology that Acton Lord and others retrieved has also been used by other characters. CAW was created using data from fallen MACE warriors. During Beige Midnight, wReamicus Maximus attached his head to a RoboMAC body in order to both destroy the Ultimate Ninja and impress Dvandom. Neither worked. Senator Binary cybernetically joined himself with a RoboMAC body, allowing him to pilot it with MAC speed.
Meanwhile, some of the nanoplague nanites within DeFacto V were shot through a dimension rift created when the time loop that led to his origin was broken. They landed in 000SUPERGUY and were found by Doctor Odd Science; those same nanites later transformed Summer.
The MicroMACs were created by beings from a faraway universe observing and copying them.
More information is available on Dave Van Domelen's site over here.