Human-form Replicators were Replicators that look like humans. They are composed of millions of tiny Nanite cells. They were first created on Hala, after the Replicators discovered their creator, Reese. Not much is known about their construction, except that Neutronium is a key element, which was why they invaded Orilla, the Asgard homeworld, as it was rich with Neutronium.
Human-form Replicators are capable of controlling other Replicators, and can only be killed with the Replicator disruptor that Colonel Jack O'Neill constructed with the full knowledge of Ancient technology downloaded into his head.
In addition, Human-form Replicators can stick their hand into the head of humans, allowing them to communicate with them in a few milliseconds and physically visit places in their memories. Fifth also showed it was possible to have both beings enter the human form Replicator's mind. As shown with Replicator Carter and Dr. Daniel Jackson, when a Replicator enters one's mind its possible to reverse it and enter the Replicator's mind at the same time. Jackson was able to do this while Replicator Carter was probing his mind for Ancient knowledge and he was able to reverse the connection. However he had the advantage of her being distracted, having the Ancient knowledge unlocked in his mind which he said helped and time to potentially experiment. (SG1: "Unnatural Selection", "New Order, Part 1", "New Order, Part 2", "Gemini", "Reckoning, Part 1", "Reckoning, Part 2")
The Human-form Replicators, being identical to the Asurans, were most likely based on a discovered Ancient design or piece of technology. The Asurans are, however, far more advanced than the Replicators in both technology, and ability. For example, as well as having the knowledge of the middle-era Ancients (Lanteans), the Asurans could also adapt to the Anti-Replicator gun in just a few minutes of exposure. It took a Milky Way Replicator hours of actually having the specs in front of it to create a working defense. The Asurans have also been around for over 10 thousand years before the Human-form Replicators (discounting time spent in the asgardian time-dilation field) . These Human-form Replicators were destroyed by Dr. Meredith Rodney McKay using a plan enacted by the Atlantis expedition with the help of the Wraith and the Travelers. (SGA: "Progeny", "The Return, Part 2", "Be All My Sins Remember'd")