There's no rule cast in stone that defines an exterior paint. You could paint the outside of your house with paint designated 'Interior Paint'. But most outdoor formulas will have a number of attributes that make selecting exterior paint preferable.
All house paints, for example, can be put into one of two categories: solvent-based or water-based. The first encompasses all oil or alkyd types, the latter is generally labeled 'latex'. It's certainly possible to use latex paint for exterior jobs. Decks or fences, for example, will sometimes get a coat of latex rather than more expensive oil-based paint.
Latex is easier to apply and much easier to clean up after, along with usually being less expensive. But in any area that gets substantial sunshine or hot weather, it won't last anywhere near as long. Also, it doesn't have a property inherent in many alkyds: chalking.
Chalking is a process in which the paint slowly oxidizes. Over time, the paint turns to a chalky dust. As rain and wind bathe the outside of the house, the chalk is gradually removed without homeowner effort. That removes dirt and old, very thin layers of paint, keeping the house looking good for a long time. Latex, by contrast, typically peels as it ages, especially from wood surfaces where it hasn't soaked in well.
Chalking may not always be desirable, either. Depending on the amount of rainfall and how the exterior surface is oriented, the chalk can remain on the surface. That requires that it be cleaned off manually. If left on the wall, the surface looks dull and old.
Oil-based or alkyd paints take longer to dry and are more difficult to spread evenly. They'll be more difficult to remove from surfaces to which they're accidentally applied, such as trim that wasn't perfectly masked. Latex paints allow easier movement of air, ensuring that any moisture trapped evaporates.
But, oil-based paints tend to be available in a range of more vivid colors. They're made with more, deeper pigments. And, they can be applied to a wide range of surfaces, of the sort usually encountered outside. Aluminum siding, for example, can be painted with a bright, oil-based paint that will last for years and continue to look great. Latex would usually peel off in a year or two.
Though many homeowners will want to use stain rather than paint, there are wood surfaces that can look equally nice painted. Non-wood surfaces are also often found outdoors. Concrete block or clay brick, stucco and other surfaces generally look better longer with an oil-based rather than water-thinned paint or latex.
Every homeowner's situation is individual and making a final decision may require some experimentation. A few dollars spent for a small amount of each type is a good investment. Spend a few hours or less to paint some small and less visible surfaces. Let the results dry for a day or two or even a year ahead and take a look. The shorter period won't reveal peeling or other problems, but it will give visual evidence to make selection easier.