A molecule, or the simplest structural unit of a substance that still keeps the properties of that substance, is a scientific word that gets used by the nonscientific, as in "Every single molecule in my body wants that chocolate cake!"
The only problem with molecules really, is that people confuse them with atoms. Here's the thing: a molecule retains the characteristic of the substance it is from, so a water molecule, which is made up of hydrogen and oxygen, is still water, while an atom — one of the tiny units that make up elements — only has the characteristics of its element, not the larger composition. Make sense? No? OK then, just remember a molecule is something really, really small, and an atom is even smaller — since a molecule is composed of atoms!