Passive transport is the movement of substances across membranes in favour of their concentration gradient, i.e., from a more concentrated region to a less concentrated region. Active transport, on the other hand, is the transport of substances across membranes against their concentration gradient, from a less concentrated to a more concentrated region. In passive transport, because it is spontaneous, there is no energy spent; the active transport, however, requires energy (work) to occur.
Active transport works to maintain or increase the concentration gradient of a substance between two regions while passive transport acts in a manner to reduce the concentration gradient.
