When you want to import an image from the Add Menu, Blender will show you two options: "Reference" and "Background," and even if they seem to look alike, they don't operate in the same way. So how could you see the difference between them?
To start, you can notice the method you use to import both images is the same: you need to search from a file browser the location where you saved the image on your computer and select it. However, once you put the illustrations into your 3D space, you will first notice the look of each one is different. On the one hand, the Reference Image will be solid, like a piece of paper, and the pivot point (which you can notice by a green highlight) will be at the center of the picture. But on the other hand, the Background Image will have the reticle of your scene captured in the illustration and will highlight the lines of the axes that cross your image.
The second main difference you will see is the way you can watch the image while you move on your scene. The Reference Image will be opaque by default and visible from both sides. In contrast, the Background Image will be transparent, which means only from the front according to the settings of your image (Only in orthographic mode, only by perspective mode, or any set you need).
All in all, the Reference Image is a practical modeling guide when creating and transforming an object or character from a drawing; that’s why you can visualize it on both sides. And the Background Image could be part of your modeling as a blueprint when 3D modeling, and that’s why you can use the standard tools to transform any object in Blender into this kind of image. Awesome! Right?