A diagram is a symbolic representation of information according to some visualization technique. Diagrams have been used since ancient times, but became more prevalent during the Enlightenment. Sometimes, the technique uses a three-dimensional visualization which is then projected onto a two-dimensional surface. The word graph is sometimes used as a synonym for diagram.

A diagram is a simplified and structured visual representation of concepts, ideas, constructions, relationships, statistical data, anatomy etc. used in all aspects of human activities to visualize and clarify the subject. A diagram can also describe phenomena, highlight correlations in certain factors or represent parts of a set.

The term “diagram” in its commonly used sense can have a general or specific meaning:
visual information device : Like the term “illustration”, “diagram” is used as a collective term standing for the whole class of technical genres, including graphs, technical drawings and tables.
specific kind of visual display : This is the genre that shows qualitative data with shapes that are connected by lines, arrows, or other visual links.
In science the term is used in both ways. For example, Anderson (1997) stated more generally: “diagrams are pictorial, yet abstract, representations of information, and maps, line graphs, bar charts, engineering blueprints, and architects’ sketches are all examples of diagrams, whereas photographs and video are not”. On the other hand, Lowe (1993) defined diagrams as specifically “abstract graphic portrayals of the subject matter they represent”.

In the specific sense diagrams and charts contrast with computer graphics, technical illustrations, infographics, maps, and technical drawings, by showing “abstract rather than literal representations of information”. The essence of a diagram can be seen as:
a form of visual formatting devices
a display that does not show quantitative data (numerical data), but rather relationships and abstract information
with building blocks such as geometrical shapes connected by lines, arrows, or other visual links.
Or in Hall’s (1996) words “diagrams are simplified figures, caricatures in a way, intended to convey essential meaning”. These simplified figures are often based on a set of rules. The basic shape according to White (1984) can be characterized in terms of “elegance, clarity, ease, pattern, simplicity, and validity”. Elegance is basically determined by whether or not the diagram is “the simplest and most fitting solution to a problem”.

Chart-like diagrams, which take a collection of items and relationships between them, and express them by giving each item a 2D position, while the relationships are expressed as connections between the items or overlaps between the items; examples of such techniques:

tree diagram, network diagram, flowchart, Venn diagram

existential graph
Graph-based diagrams; theses display a relationship between two variables that take either discrete or a continuous ranges of values; examples:

histogram, bar graph, pie chart, function graph, scatter plot
Schematics and other types of diagrams, e.g.,

train shedule diagram, exploded view, population density map, Pioneer plaque

Three-dimensional diagram
Many of these types of diagrams are commonly generated using diagramming software such as Visio and Gliffy. Thousands of diagram techniques exist. Some more examples follow.

Diagrams may also be classified according to use or purpose, for example, explanatory and/or how to diagrams.

In mathematics, a diagram is a diagram used to represent objects and to support the reasoning.

Venn diagrams or Euler diagrams are used to represent sets and their elements. The elements appear as dots or small crosses, surrounded by a closed curve that forms the whole. In the case where the set contains too many elements, only the whole is represented by the part of the inner plane to the curve.
Caroll diagrams are used to represent sets. A set E is represented by the portion of the plane delimited by a square and a subset of E is represented by regions obtained by sharing this square with lines parallel to the sides of the square.
Cartesian Diagram or Relationship Diagram. Let E and F be two sets and R a relation from E to F. The elements of E are represented by points on a line, and the elements of F are represented. by points on a line perpendicular to the first. When an element x of E is linked to an element y of F by the relatio R, a cross is marked at the point of abscissa x and of ordinate y . These diagrams are used in particular to graph functions.
Sagittal charts are used to represent relationships. We represent two sets E and F by a Venn diagram. When an element of E is related to an element of F for a relation R, we join x to y by an arrow of origin x and of goal y.
Hasse diagrams are used to represent a finite ordered set.

A diagram of Feynman makes it possible to visualize the interactions between particles (example: a proton can form a neutron and a pawn which will re-annihilate soon after).
Bode diagrams and Nyquist diagrams represent the frequency response of a system and allow the study of the stability of the latter.
Fresnel diagram.
The Sillen Diagrams allow the representation of the logarithm base 10 of the concentration of chemical species of an element as a function of pH.
The diagram is the angle of rotation of the crankshaft during which a phase of the cycle of the 2-stroke engine takes place, therefore there are the intake diagrams and the exhaust diagrams. If the intake diagrams are 130 ° the intake transfers are open for 130 °; the crankshaft then opens the transfers for an angle of 130 °.

In archeology, the diagrams used are the stratigraphic diagrams used to represent the relations of anteriority, posteriority and contemporaneity of the stratigraphic units determined at the time of the excavation. They are also used to determine the aggregations, that is to say the sets of stratigraphic units corresponding to the same set. They are also main tools for the interpretation of events occurring on the site.

Source From Wikipedia