Bonds between atoms with differing electronegativity are polar: the atom with the higher electronegativity has more electron density around it, and thus a partial negative charge: the other atom has less electron density and thus a partial positive charge. The only non-polar bonds are those between the same atoms: H-H bonds, for example: all others are at least slightly polar, with the polarity increasing as the electrnegativity difference increases.
The Chime models below give a graphical model of this phenomenom. The density of dots around the nucleus indicates electron density: areas with higher electron density have more dots. Compare the H₂ molecule (nonpolar) to the HF molecule (polar) to the LiF molecule (strongly polar) As the electronegativity difference increases, more and more electron density moves to the more electronegative atom, making the bond more polar.

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