Molecular clouds are dense regions in space filled with gas and dust, primarily composed of hydrogen molecules. These clouds are crucial for the formation of stars and planetary systems, as they provide the necessary material for gravitational collapse. They are often found in the spiral arms of galaxies and can be quite large, sometimes spanning hundreds of light-years. Within these clouds, temperatures are low, allowing molecules to form and survive. The dense areas within molecular clouds can lead to the birth of new stars, making them essential for the ongoing cycle of star formation in the universe.