Nanoprobes are microscopic robotic devices used by the Borg for the primary purpose of assimilation. Injected into a target's bloodstream via assimilation tubules, the nanoprobes immediately begin to take over the host cells' functions. Nanoprobes can also be modified for a variety of medical and technical tasks. (VOY: "Scorpion")
When Borg drones attempted to assimilate Doctor Phlox of the Enterprise NX-01, his Denobulan immune system was able to fight off the nanoprobes long enough for him to discover that they were vulnerable to omicron particles, although the scale of the radiation dose required to destroy every nanoprobe in his body means that this treatment cannot be used against the assimilation process on a larger scale. (ENT: "Regeneration")
In 2374, The Doctor was able to modify nanoprobes as an offensive weapon against Species 8472. He reprogrammed them to mimic the alien cells' electrochemical signatures, so that they could evade detection by Species 8472's immune system while continuing to assimilate. These nanoprobes could be delivered inside a photon torpedo or along a phaser beam. (VOY: "Scorpion", "Scorpion, Part II", "Prey")
Nanoprobes can also be modified to attack specific cells which contain a virus or some other disease and so help the patient recover.
In 2375, Seven of Nine encoded some of her nanoprobes to assimilate the synthehol molecules in Kadi Ambassador Tomin's bloodstream, allowing him to recover from his intoxication in time to return to his people. (VOY: "Someone to Watch Over Me")
Borg nanoprobes are worth six bars of latinum each on the open market. In 2377, Ferengi marauders attempted to obtain Seven of Nine's nanoprobes by perpetrating an elaborate scheme, in which a reprogrammed hologram of Reginald Barclay would bring the USS Voyager to them through a geodesic fold, with the effect of killing the entire crew due to the radiation they were exposed to while traversing the fold. (VOY: "Inside Man")