Diagrams tell you the information about the object, and how it looks, or works.
Diagrams are basic pictures that show the parts of something using labels and arrows to explain the different parts of an object. Basic pictures or line drawing are used becuase it is easier to label the parts, and make it easier to see the labels. For example, in the slide below shows a simple drawing of a tree with 4 parts.
The labels on a diagram show the name of the part or how it works. The label on a diagram can be on top of the picture and on top of the part. Or it can be beside the picture pointing to the part. For exmaple, in the fourth slide, the labels for the dog are beside the picture, but they have arrows pointing to the part.
A digram shows how something looks and works. Diagrams have labels naming the different parts of the object.