What is Polar and non-polar molecules?

Example 30.7 (Four molecules)

Hydrogen chloride $\ce{HCl}$ has one polar bond and is polar, chlorine $\delta^-$. Ammonia $\ce{NH3}$, a pyramid of three polar $\ce{N-H}$ bonds, is polar, nitrogen $\delta^-$. Methane $\ce{CH4}$ has only nearly non-polar $\ce{C-H}$ bonds: non-polar. Tetrachloromethane $\ce{CCl4}$ has four polar $\ce{C-Cl}$ bonds, but at the corners of a regular tetrahedron: their effects cancel and the molecule is non-polar.