Answer: Nanotechnology is the branch of technology that covers the manipulation of matter in the range for 1 to 100 nanometers (where 1 nanometer is equivalent to 1x10^(-9) meters)
One important thing of the nanotechnology is that in this scale size the effects of the quantum mechanics start to be relevant, wich makes this a whole different thing that the technologies that we know.
You also may notice that the description of the branch is very broad, fields that may enter in it are: Molecular enginering, surface science, molecular biology, etc.