Polar molecules form, when one atom has a stronger attraction to electrons than the other, or when there are electrons that are not bonded. One end is positive, the other negative, so that they will be attracted to each other and anything else with a charge.
The reason for hydrogen bonds is the polarity of the molecules. Polar molecules have their boiling points much higher than the molecular mass alone suggests. Water would boil at -100 C without the polarity of the molecule. The polarity is also the reason why liquid polar solvents are wet: the electric forces make the molecules stick to things.
One consequence is that a water jet (from a faucet) bends around a plastic comb, which is electrostatically charged by rubbing it to a woollen cloth. Mitsubishi Heavy Industries Corporation has drafted plans for an entire ferry, which moves using this phenomenon.
Polar substances usually dissolve to water, which is a highly polar substance. The polar solvents have so strong an attraction between their molecules that no nonpolar molecules can stay in their way. This is why nonpolar substances, like oil, don't dissolve in water. The strong attraction between the water molecules keeps them together and forces the oil molecules out, because they cannot break the strong hydrogen bonds to get into the water. The electric charge will cause even nonpolar substances to attract the polar molecules strongly, because the polar charge strengthens the van der Waals forces. You can see this, if you put a drop of oil on water: it will spread to a thin film. The forces keeping the oil drop together are smaller than the attraction to the polar water, so that the drop is attracted to the water. The area in contact with the water is maximised in a thin film.
Polar and nonpolar solvents don't repel each other. That is Bad Science, which even many chemists believe in, caused by false analogies and dumbing down in the name of good communication. More about this popular misconception can be found from Dr. Kevin Lehmann's Bad Chemistry page:
Water is polar, because the oxygen molecule has eight electrons (an octet), but only two pairs of them are shared with the hydrogen atoms. The rest, the four electrons, form two pairs. (They pair up because one electron has its spin "up" and the other "down".) The two pairs are as far away as possible from the hydrogens and from each other. On a sphere, this means that they are in the vertices of an imaginary tetrahedron. Now you can imagine a tetrahedron with two vertices that have a negative charge. This means that there is a electric potential difference between the ends - the other side is positive, the other negative.
A side view of a water molecule:
:(+) __ (+)
: / \
: / \__/ \
: / \
: | |
: | (++) |
: | |
: \ /
: (-) (-) (-)
Polar molecules have one atom with a stronger electronegativity than the other. Hydrochloric acid is one, but it is a gas when pure at rtp. The chlorine atom has a higher electronegativity, so it "robs" some electron density from the hydrogen. Again, there is a potential difference between the ends. All compounds are a little polar, because there is always a difference in electronegativity. For some compounds, like oil, the difference is so small that they're almost nonpolar.
Thanks Professor Pi