Pre-Main Sequence Stars are young stars that are still in the process of forming. They begin as clouds of gas and dust, known as molecular clouds, which collapse under their own gravity. During this phase, they gather mass and heat up, eventually forming a dense core that will become a star. These stars are not yet in the stable phase of hydrogen fusion that characterizes Main Sequence Stars. Instead, they exhibit various stages of development, such as T Tauri stars and Herbig Ae/Be stars, and can be found in star-forming regions like nebulae.