Nanites are microscopic machines which can enter a person's body. They can be used for a variety of uses.
Nanites by race
During the war with the Wraith, the Ancients used nanite technology in an attempt to destroy the Wraith from the inside. They gave the nanites the ability to self-replicate, and installed a code giving them an aggression surpassing that of the Wraith. To protect themselves, the Ancients installed a law into their base code that prevented them from harming anyone with the Ancient Technology Activation gene. However, the nanites instead took human form, becoming the Asurans. The Ancients eventually concluded that the Asurans would never become the weapon they wanted, and tried to destroy them. However, their efforts failed; some of the Asurans survived, and they rebuilt their civilization and attacked the Wraith soon after the Ancients retreated to the Milky Way Galaxy. The Asurans (Replicators) were later destroyed by Dr. Rodney McKay with help from the Wraith and the Travelers. (SGA: "Hot Zone", "Progeny", "Be All My Sins Remember'd")
Seeking to learn what humans would evolve into, the Goa'uld Pelops used nanites to accelerate the human lifespan to run its course in one hundred days. On the planet Argos, Pelops confined the humans to a small area, as the nanites needed to be within reach of a device which would send out instructions to them, making them accelerate the life-cycle while the Argosians were sleeping. Colonel Jack O'Neill was infected with the nanites, causing him to age to 100 in a matter of weeks. However, after the Argosians destroyed the statue of Pelops containing the device, the nanites were destroyed by the immune systems of the humans, allowing them to age at a normal rate. (SG1: "Brief Candle")
Pelops also used these nanites to experiment with human evolution on the planet Tiryns. In contrast to the idyllic conditions the Argosians were exposed to, the Tirynsians were planted on a world with toxic conditions, which were artificially worsened every dozen years. This caused the Tirynsians to grow little food and live in a poisonous atmosphere, but through evolution they adapted. (RPG: "Fantastic Frontiers: Stargate Season One")
The Orbanians used nanite technology as a means of educating their people. Certain children, designated Urrone, would be selected to have their nanites removed from their body, allowing the rest of the Orbanians to learn what they had learned. However, the removal process rendered the child mentally an invalid. Attempts to reintroduce the nanites failed, and the children were taken to secure facilities where they were well cared for, but no efforts were made to re-educate them. Eventually, after learning that there was more to life than learning, the Urrone Merrin allowed her nanites to be removed, which also implanted a part of her personality onto the Orbanians, who then decided to teach other Urrone the "old fashioned way". These nanites are made from Trinium, silicon and one unknown mineral. (SG1: "Learning Curve")
When the Replicators arrived on Hala and found Reese, they hailed her as their creator, and found her nanite cells possessed certain advantages that regular Replicator blocks did not, including resistance to Tau'ri guns. The Replicators created five Human-form Replicators, possibly based on some plans they found for the Asurans. These Replicators could probe the minds of individuals and learn vital information. Replicator nanites require Neutronium to function. (SG1: "Unnatural Selection", "New Order, Part 1", "New Order, Part 2")