Backpacks come in all shapes and sizes and are used for a multitude of tasks, there are Tramping / Trekking / Hiking packs mainly used for multi day trips and expeditions where more gear needs to be carried, there are Alpine and Adventure Racing packs which are generally constructed from lighter materials to ensure faster movement. Travel packs designed for just that, day pack's and also hydration packs to aid in ensuring your fluid intake is sufficient.