While the ingredients will differ from brand to brand, most glass cleaners contain a number of the same active ingredients. One of these is isopropyl alcohol, also known as rubbing alcohol. This is an extremely weak acid which is very effective when it comes to dissolving grease and oil on contact. It also evaporates quickly, leaving you with a streak-free shine.
Many glass cleaners also contain 2-Butoxyethanol - a glycol ether that has good properties as a surfactant - which helps to loosen dirt from surfaces. This substance is also what gives window cleaner its subtly sweet odour.