The main difference between perennial herbs and annual herbs is their lifespan. Perennial herbs are plants that live for more than two years, often returning year after year. They have a longer life cycle and can continue growing and producing for multiple seasons. On the other hand, annual herbs complete their life cycle within one year. They grow, flower, produce seeds, and then die off at the end of the growing season. They need to be replanted each year for continuous cultivation.