Dipole-dipole interactions occur between polar molecules, which have a positive end and a negative end due to uneven distribution of electrons. These interactions arise when the positive end of one molecule is attracted to the negative end of another, creating a force that can influence the physical properties of substances, such as boiling and melting points. This type of intermolecular force is stronger than London dispersion forces but weaker than hydrogen bonds. Dipole-dipole interactions play a significant role in the behavior of many substances, including water and ammonia, affecting their solubility and reactivity in various chemical processes.