In theory any photo can be stock, but successful stock photography tends to have the following attributes:

1. It can be used to sell a product or concept (or many different concepts)
2. It is bright, colourful and appealing
3. It is designer-friendly: it can have text superimposed on it, or it can be easily composited with other photos or designs
4. It is aimed at a particular market (business, tourism, industry, news media, etc)
5. It is generally technically perfect
   • pin-sharp focus in the appropriate part of the image
   • no deliberate blur, roughened edges, selective colour, black & white conversions or vignette, to give the customer the choice to modify as they see fit

Some traditional stock images:

• A good-looking businessman or woman smiling, usually in a generic office environment. This can be used to sell many different products or ideas (e.g. professionalism, customer service, teamwork).
• A green healthy leaf with some droplets of water (environment, health, purity, food)
• A landscape shot of a popular tourist destination (tourism, local culture, nature)

In contrast, fine art shots tend to be more subtle and less obviously targetted. They can be imperfectly shot if this helps to convey the desired idea or mood.

There is definitely an overlap. However, I personally believe there is a growing interest in stock buyers for less obvious imagery, so the overlap is likely to grow.