Polar compounds are compounds that have slight charges (slightly positive and slightly negative) within the compound. The slight charges are due to electronegativity differences of .3 to 1.4. (Electronegativity is defined as the attraction an element has for electrons- the greater the electronegativity, the more electrons are pulled toward an element.) Examples of polar compounds would be: water (H2O), hydrogen fluoride (HF), hydrogen chloride (HCl), and ammonia (NH3).
Nonpolar compounds are compounds that do not have slightly negative and positive charges within the compound. The electronegativity differences of nonpolar compounds are between 0 and .2. All hydrocarbons without major subgroups are nonpolar compounds.
Sometimes the electronegativities within a compound are such that one would classify the compound as being polar covalent, yet the compound acts nonpolar because the compound is symmetrical. In such a situation, the outward pulling in opposite directions with equal forces cancel each other out.