Molecule
A molecule is a group of two or more atoms held together by covalent bonds that acts as a single unit, such as water or oxygen gas.
Definition
A molecule is a group of atoms stuck together that moves around as one piece, and it can be made of the same kind of atom or of different kinds. It is the smallest particle of a substance that keeps that substance's chemical properties, with two or more atoms joined by covalent bonds; diatomic elements such as $H_2$, $O_2$, and $N_2$ exist as molecules of one element, while molecular compounds like $CO_2$ and $C_6H_{12}O_6$ contain more than one. A molecule is electrically neutral and has a definite structure: its geometry is predicted by VSEPR theory and described by bond lengths and bond angles, and its polarity depends on both bond polarity and shape. Molecular mass is the sum of the atomic masses of its atoms, and one mole of molecules contains $6.022 \times 10^{23}$ of them.
Example
A water molecule is two hydrogen atoms and one oxygen atom joined together, and an oxygen molecule in the air is two oxygen atoms joined together. Glucose has the formula $C_6H_{12}O_6$, so one molecule contains $6$ carbon, $12$ hydrogen, and $6$ oxygen atoms, $24$ atoms in all, and breaking any of those bonds leaves something that is no longer glucose. Water is bent with an $H$-$O$-$H$ angle of $104.5^\circ$ because oxygen's two lone pairs push the bonds together, which makes the molecule polar; carbon dioxide is linear ($180^\circ$), so its two polar bonds cancel and the molecule is nonpolar, which is why $CO_2$ is a gas at room temperature while water is a liquid.
Key Insight
A molecule can be an element or a compound, but a molecule is not the same as a compound; oxygen gas is molecules but not a compound. Ionic compounds like salt do not form molecules at all, they form giant repeating crystals, so "a molecule of salt" is not a real thing even though people say it. Molecular shape controls function: enzymes recognize substrates by fit, drugs bind receptors by shape, and a mirror-image molecule can have completely different biological effects even with identical atoms and bonds.