These are smaller versions of buses that can ferry passengers for terminal to terminal. Depending on where the minibuses are operating, they are either used for short distance or for longer land travel, particularly in some far-flung areas where any other means of transportation are few and/or limited. They can carry people, livestock and other merchandise. Minibuses are much cheaper, do not have air conditioning and seats are small and spaced close together, had my experience going from Infanta Quezon to Paete.
More fun and transport in the Philippines.