Active transport is mediated by specific (or relatively specific) transporter proteins and drive substances across membranes "against" a concentration gradient. Passive transport is not mediated by anything other than normal diffusion. Active transport allows substances to be moved from regions of low into regions of higher concentration; this is "backward" from the way they would move if only passive transport were possible. Active transport is almost always coupled to ATP hydrolysis as it takes energy to move something against a concentration gradient, whereas passive transport does not need anything beyond chemical potential as material is "falling down" the gradient.
