Molecular clouds are dense regions in space filled with gas and dust, primarily composed of hydrogen molecules. These clouds are crucial for star formation, as they provide the necessary material for stars to develop. They can be found in various sizes and masses, often appearing as dark patches against the backdrop of brighter stars and galaxies. The temperature within molecular clouds is typically very low, allowing molecules to form and survive. These clouds are often the birthplace of new stars and planetary systems, making them essential components in the life cycle of the universe.