Hydrogen chloride is a polar molecule because it contains a polar, covalent bond between hydrogen and chlorine. This gives the hydrogen a partial positive charge and the chlorine a partial negative charge.
Hydrogen chloride is formed when one of the valence electrons from chlorine and hydrogen's electron are shared between the two atoms to form a covalent bond. The electrons in a covalent bond are not equally shared between the two atoms. This is because chlorine is more electronegative and pulls the bonded electrons closer to it. This leaves hydrogen with a partial positive charge and chlorine with a partial negative charge, thus making the molecule polar.