There are two common terms used to describe the area that holds our attention in a painting: area of interest and focal point. For many, these may mean exactly the same thing, but I attach slightly different definitions to each to clarify their purposes. The "area of interest" is the place within the composition to which an artist wishes to direct their audience. Focal points are areas of lesser interest that lead the viewer through the composition, supporting the main area of interest. Think of it as it relates to movie production: You are the producer/director in search of the star (area of interest) and supporting cast (focal points). The star will carry the weight of the production and the supporting cast will support and flatter the star's role. The remainder of the roles will be filled with bit players and extras. Everything has to work in harmony to create a successful outcome—one that leaves the audience with the message you hope to convey. Now adapt this scenario to your painting, planning an area that is the main interest and then lesser points that allow movement and support to the area of interest. My plein air painting, Evening on the Malheur (pastel, 12x16), for example, has a strong area of interest around the tall bush, and focal points at middle right and lower left.
 
Understanding how the human eye sees is helpful in learning how to handle the area of interest and focal points. Our eyes, working with the mind, focus on one given area and everything else falls into a soft blur. Only when we move our attention to another area does it become sharper. Since we have taken in a lot of detailed information throughout our lives, our mind quickly associates this knowledge to a symbolic representation. In essence, we believe we see things that aren't really there, because we know they are. Think of a tree. Since we know there are individual leaves on a tree, we believe we can see them even when we're not focused on the tree. This often leads us to place more information than is needed in areas that are not the focus of the painting. Even a highly detailed painting has to employ a degree of heightened interest in one area; otherwise, it may become confused.

Wherever we direct our eye has the sharpest focus; therefore, that is where the greatest contrast (or focus) will occur within the painting. When things are in focus we're able to delineate the edge, value and color differences. As things fall away from that focused area, they become slightly weaker with less contrast. I use an order of importance when establishing the area of interest and focal points: first, sharper edges, followed by value contrasts (whiter-whites and darker-darks), and finally, stronger color saturation (brighter-intense-colors). These tools represent the vocabulary of the visual language we use when painting. How we arrange and manipulate them is how we communicate to our audience.
 
If you wish to learn more about composition and design, I recommend the book, A Painter's Guide to Design and Composition (North Light Books, 2006), by Margot Schulzke. In this book, I and a fine assortment of fellow painters discuss our individual approaches to arranging a painting.