A Cyborg is a human or other life form that has been modified with machinery and metal for better functioning. These mechanical enhancements can include cybernetic replacements for missing organs or body parts, metal skin for extra protection, and weapons installed somewhere in the body.
Cyborgs have mechanical parts, which are usually visible on the outside. The Pacifista appear to have no visible mechanical parts at all but some are seen hidden underneath their clothes. Aside from this, they may appear as a normal human or animal on the outside. Modifications may be more apparent when they receive wounds as they may leave their mechanical parts exposed.
In the One Piece series, cyborgs are rare, as the technology required for their creation is somewhat advanced.
Though they have mechanical parts, so long as their brain is not altered in any way, these cyborgs maintain their free will. As shown with Bartholomew Kuma, cyborgs with their brains modified lose their free will. This leaves them "dead", reducing them to being nothing more than a tool. It is also claimed by Donquixote Doflamingo that such cyborgs have no memories or recollections of their past lives.
Indeed, Kuma has become obedient to the words of the World Government he once hated so much. In an earlier instance when Luffy hears a "heartless" remark from Franky, he asks him if he replaced his heart, to which he apologizes in metaphoric terms that he "never meant to go that far". This confirms Franky, unlike Kuma, has not lost his humanity.
They can also be made from the corpses of the dead as well as the bodies of the living. The one thing they all share in common is they are still capable of bleeding.
Their mechanical parts put them at high advantage against normal humans, their strength is normally far greater than a normal humans and each cyborg is essentially a living weapon. Those who are cyborgs often have an advantage in battle, as they are more powerful than regular humans in both offense and defense.
Even cyborgs are not without certain flaws that leave them vulnerable, since the modifications that they have received are generally man-made, there are usually some limits to the extent of what they can do. For example, Franky modified his right arm to function somewhat like a bludgeoning tool while modifying the left to house several gunner-type weapons. He could not possibly interchange both functions in both arms with each other whenever he feels like it. And because he made the modifications personally on himself, there were several restrictions when he converted his body into a cyborg, namely the place around his back area where he wasn't able to reach around with his arms, leaving the said location lacking with metallic reinforcement in comparison to the rest of his body.