Of course, the difference between concept art and illustration is blurry. But let me try to explain where I draw the line.
My main work happens at the beginning of projects - visualizing ideas and testing directions. These images facilitate a vision for a product that has yet to be developed. They are an important tool in the preproduction phase and their main purpose is to communicate conceptual thoughts between people. This is considered concept art.
On the other hand, there are artworks that are created to express a certain feeling. Or there are projects where I am commissioned to deliver a final painting. This type of image is usually for permanent usage and the end product itself. That kind of artwork I consider an illustration.