Replicators are seemingly unstoppable 4 legged spiders, comprised of small "blocks".
Replicators are generally built for the sole purpose of replicating, they do this by using every usable material they can aquire, and transform it into a replicator "block"
Replicators use these "blocks" to build more of themselves, aswell as, walls, engine upgrades, and weapon upgrades. At one point they had even built a ship out of them.
During a war with the Asgard, the Replicators ended up on Hala, the Asgard homeworld, where they found Reese. Using Reese as a blueprint, Replicators created Human forms of themsevles, comprised of small nano-bots. Replicators have total control of these cells, and are able to go through solid objects, or turn their arms into weapons.
They were eventually killed by the weapon at Dakara, but.. the first, and true Replicators live on in the pegasus galaxy.
Replicators beat the hell out of the Goa'uld, Humans, Asgard, and were orignally built to kick the Wraiths ass. How much better do these little spiders get?