Admittedly, "Illustration" and "Concept Art" are some pretty vague terms.

 

Illustration is an all-encompassing label that means different things to people in different industries. Whatever. You need pretty pictures to help communicate your ideas? We have you covered, regardless of art style, medium or delivery method. For the sake of clarity we'll call any finished, client-facing artwork an illustration.  
 

Behind the scenes, we have Concept Art. This term refers to all the fun conceptual work that the public may never see, but that is crucial to world-building for all IP. Including things like character, environment, vehicle or prop designs, and even some of the "sketch art" and storyboards featured here