The difference between graphic design & illustration must be known to both the designer or illustrator . You can not offer illustration work to a designer or graphic design work to a illustrator unless the professional has expertise in both the fields. So, what exactly is the difference between the two?
Here is example of difference between graphic design and illustration:
Illustration is about creating images that do not require attention from the viewers to a specific object on the canvas. The viewers see whole of the canvas and not a particular picture or image. These images are created to work with something. So, the trick is in illustrating in such way that while there is no need for direct attention to the whole of the illustration, the viewers still do not distract from that particular thing. Role of the illustrator is to add character to the main thing in a way that the viewers only notice it or not focus on it.
On the other side, graphic design is about creating images that demand direct attention from the viewers. The designer makes the images with the sole purpose of bringing the focus of the viewers to the drawn object. All design elements are employed for this purpose only. All of the design elements incorporated in a graphic design compete with each other for attention. But an experienced designer knows which elements should get focus first and which ones should get noticed in the end.
So, find out a professional who is expert in both graphic design and illustration if you need to make an image that carries both the features.