Stars can be classified into several types based on their size, temperature, and brightness. The main categories include dwarfs, giants, and supergiants. Dwarf stars are smaller and cooler, while giant stars are larger and hotter. Supergiants are the largest and most luminous stars in the universe. Another classification system is based on temperature and color, which includes O-type, B-type, A-type, F-type, G-type, K-type, and M-type stars. O-type stars are the hottest and blue, while M-type stars are the coolest and red. This classification helps astronomers understand the life cycles and evolution of stars.