I've just finished a commissioned painting, based on a study.  What is the difference between a study and a painting?  I can only speak for my own style of working, but in general, both are original oils, but my studies are small, loose and are often the first stage in creating a larger work, which is more detailed.

For a perfect example here is a 6"x8" study

And here is the 36"x48" painting I did based on that small, loose composition:

The size gives me room to add details.  Both have a sense of realism while being painterly, I just use larger brushes on the larger paintings.

Studies are the best way to test a composition, and I often use this when working with clients on custom oil paintings, making sure they get exactly what they want.   In this case I mixed the background colors to coordinate with the walls of a newly redecorated room, by using the paint swatch sample sent to me by my client.  

The study is called "Early Spring", my client and her friends started referring to this as her "Moo Baby".  As soon as it's delivered and on the wall I'll see if I can get a photo to share.