Definition of diagram

From American Heritage Dictionary:

1. A plan, sketch, drawing, or outline designed to demonstrate or explain how something works or to clarify the relationship between the parts of a whole.

2. A graphic representation of an algebraic or geometric relationship.

3. A chart or graph.

From Merriam Webster Dictionary:

1. A graphic design that explains rather than represents; especially a drawing that shows arrangement and relations (as of parts).

2. a line drawing made for mathematical or scientific purposes.

Definition of structure

From American Heritage Dictionary:

1. Something made up of a number of parts that are held or put together in a particular way.

2. The way in which parts are arranged or put together to form a whole; makeup.

3. The interrelation or arrangement of parts in a complex entity.

From Merriam Webster Dictionary:

1. The action of building; construction.

2. Something (such as a building) that is constructed.

3. Something arranged in a definite pattern of organization; leaves and other plant structures.

4. Manner of construction; makeup.