The two factors that determine whether or not a molecule is polar are if the individual bonds are even and the shape of the molecule. If the molecule is perfectly symmetric, the molecule will not be polar even if there are polar bonds present.
Polar bonds are formed when one atom in the bond has a much stronger pull towards electrons than the other atom. The difference in strength can be predicted by comparing electronegativity values. If one electronegativity value is higher, that atom will pull the electron closer and develop a slightly negative charge, while the other atom develops a slightly positive charge.