Hydration bladders are portable water storage systems designed for outdoor activities like hiking, biking, and camping. They typically consist of a flexible reservoir that can hold several liters of water, along with a tube for easy drinking. Users can carry them in backpacks or attach them to gear, allowing for hands-free hydration. These bladders are often made from durable, lightweight materials and are designed to be leak-proof. Many come with features like wide openings for easy filling and cleaning, as well as built-in filters to ensure safe drinking water. Popular brands include CamelBak and Platypus.