Interior latex paint is used exclusively for indoor applications, while exterior latex paint is used solely for outdoor applications. Interior and exterior latex paint have different chemical properties, but they do not differ all that much.
Interior latex paints stand up better to abuse, such as cleaning, scrubbing and staining, as they are formulated to be more resistant to physical damage. The resins used for interior latex paints can be firmer, because the paint does not have to deal with weather and temperature differences.
Exterior latex paints are more resistant to mildew and fading, while not necessarily being stronger or more resilient than interior latex paints. The chemicals used to create exterior latex paints must allow for the paint to be flexible so that it may stand up to moisture and temperature changes.