Nano-particle or Molecule? What's the difference between something being labeled a "nano-particle" or it being called a "molecule"?
 A: It's not simply a matter of size. Generally nanoparticles are a few nm to 100 nm and most molecules are smaller. But, for example, single chain of a high molecular weight polymer or a DNA molecule can easily be much larger than 100 nm which would put it outside the conventional "nano" range.
The distinction is somewhat fuzzy. Is a fullerene a molecule or a nanoparticle? silsesquioxanes? Why is polystyrene a "molecule" and a carbon nanotube a "nanoparticle"?
In this "gray area" it seems to come down to how it behaves, and how it's used. If the thing behaves in a similar (qualitatively) way as larger particles, and can be used in a similar way, you usually call it a nanoparticle. If the thing sticks together to make a solid or a liquid material, you usually call it a molecule. 
A: It has to do with size. A molecule is in the range of picometers. A carbon-carbon bond is 154 picometers, so you expect most molecules to fall within the range 100-1000 picometers = 0.1 to 1 nanometers. 
A big molecule, such as a ribosome, or DNA, falls in the range of 10-1000 nanometers. Everything that falls in this range is "nano" by very nature. Nano-particle can refer, for example, to a cluster of gold atoms in the nanometer range.
The fascination about nanotech is the convergence between the bottom-up approach of synthesis chemists do (such as building large molecules from smaller ones) vs. the top-down approach physicists do (such as making smaller and smaller transistors on a chip). The encounter point is in the realm of nanotechnology, a realm where nature makes all its wonder happen since billion of years, and we are just starting to move the first steps.
A: A nanoparticle as typically used in nanotechnology refers to a particle with diameter on the order of 1-100 nanometers, or $10^{-9}$ to $10^{-7}$ meters.  However, it's not just a matter of size.  These particles are not typically "molecular" in the sense that they are not stoichiometric units made out of atoms held together by covalent bonds.  Indeed, most nanoparticles are made out of very tiny pieces of crystals or metals.
Stefano Borini gives the rough size of molecules, for a bit more detail, atoms can vary from 50-500 picometers, or $5\times10^{-11}$ to $5\times10^{-10}$ meters (a picometer is 1000 times smaller than a nanometer!) in diameter; small molecules (2-10 atoms) typically are on the order of 1-100 angstroms, or 100 picometers or so.  For instance, benzene (C6H6) has a side length of 140 picometers.
A: A molecule is the collection of atoms all connected by covalent bonds, while a nanoparticle is any kind of particle1 with a size in the nanometer scale, that is (usually) between 1-999 nm.
Also,  

Although the size of most molecules would fit into the above outline, individual molecules are usually not referred to as nanoparticles.

1)For example crystals and amorphous bulks
A: A nanopaticle is collection of atoms or molecules that is less than 100 nm in any dimension and has a special and controllable function according to the molecular blocks of it. 
