Symmetry is the equality in size, shape, and/or position between parts or elements or objects. 

Three kinds of symmetry (or balance) are: symmetrical, asymmetrical, and radial.

Symmetry is the way in which one part of something duplicates another part. There are different types of symmetry, but when talking about symmetric balance, what is usually meant is bilateral symmetry, where one side or half of something is the mirror image of the other half. 

Symmetric balance is also called formal balance, as it tends to lead to stable, orderly designs that are less dynamic than asymmetric works.

Near symmetry, where two halves are not exactly the same but still balance, is a variety of symmetry commonly used in art and seen in nature. 

In art, small differences between the two halves of something can be used to create more visual interest and liveliness and to lessen rigidity. Larger differences will create asymmetry, which may or may not balance.

Emphasis is created by visually reinforcing something we want the viewer to pay attention to

Focal points are areas of interest the viewer's eyes skip to. The strongest focal point with the greatest visual weight is the dominant element of the work. Elements of secondary importance could be termed sub-dominant, and elements with the least visual weight subordinate

Isolation, leading lines and convergence, contrast, anomaly, size, placement, framing, focus and depth of field, and absence of focal points are some of the strategies used to help create these degrees of importance.

Isolation 

• Separating the subject from other distracting elements and placing it against a plain background, the viewer is left with nothing else to focus on. 

• Photographers pay special attention to what is in the background of the picture and work to eliminate unwanted clutter by changing the viewpoint the photo is taken from, or by masking or moving/removing things from view

Leading Lines and Convergence 

• A line, arrow, or similar triangular or elongated element can indicate a direction and point towards something, leading the eye in that direction. When multiple elements converge toward a point (such as lines going back into perspective), they can create an even greater pull of attention in that direction.

Contrast 

• The more something contrasts with its surroundings, the easier it is to see and the more energy it will seem to have (eg., strong contrast in value and colour can make elements "pop")

Anomaly

• A single square in a repeating pattern of circles will stand out because it's different and doesn't blend in, it breaks the pattern: this can call attention and add interest. 

• In something perfectly flat, smooth, white, etc. our eye will always be drawn to the one little flaw - the pencil scratch, the rough spot, the wrinkle. 

• Anomaly can also be created by juxtaposing things that are not normally seen together, or depicting scenes that invert or alter the everyday.

Size - The larger it is, the more visual weight it will have and the more clearly it will be visible. Do not be afraid to fill the frame with your subject, even if it means not all of the subject will fit in the frame. Concentrate on and enlarge what is important, so the viewer can connect with it.

Placement - An element placed front and center will confront the viewer. All else being equal, a third of the way in from any border can be a visually pleasing place to locate something of importance.

Framing - Just as a picture frame sets off a painting from the wall and calls attention to the artwork, similar elements within an artwork can help direct attention. Posing or painting a figure framed by a doorway, window, objects associated with them (a cook framed by hanging pots and pans, etc.) or other environmental element can help set off the subject and tell something about it. In three-dimensional work, framing (tubes, tunnels, portals, openings, gaps, cracks, etc.) can not only reveal or limit views, they can also let in and channel light, wind and rain, creating other interest.

Focus and Depth of Field - When we look at something, we focus on it so we can see sharp detail. In art, areas depicted in sharp focus will be dominant. Depth of field is the range from near to far in which objects appear to be in focus. When you look out at a landscape, for example, everything from pretty close to far far away appears in focus (if your vision or glasses are good) and the depth of field is said to be large (deep focus). In this case, other factors than focus will direct attention within the work. If you are looking at someone's face close to yours, you may only be able to focus on their eyes, while everything else is blurry - this is called shallow depth of field, or shallow focus. Using a shallow depth of field gives the artist the ability to direct attention by choosing just what she wants to be in focus, be it near-distance, middle or far. Blurring the background can be a good way to reduce distracting elements and help isolate the subject.

Absence of Focal Points - If a work has no real focal points, then the work as a whole becomes the focal point.

A principle of design. The combination of elements so as to highlight their similarities and produce a unified composition.

A principle of design. The use of recurring elements to direct the movement of the viewer's eye through the art work and give a sense of unity to the composition. 

There are five kinds of rhythm:  random, regular, alternating, progressive, and flowing.

Rhythm is like pattern, in that the same elements (i.e. shape, line) are repeated; however, with rhythm there are slight variations in the pattern.

Rhythm is the effect of regularly spacing visual elements (i.e. lines, shapes, colours) just like the beat in a piece of music. It does not have to be continuous, so long as it repeats, so 1–2–3, 1–2, 1–2–3, 1–2 is a rhythmic pattern just as much as 1–2–1–2–1–2–1–2.