Composition and Balance are extremely important in an artwork. There are a variety of ways to achieve this in an artwork.
Composition is the arrangement of things in art.
Balance is the comfortable or pleasing arrangement of things in art.
There is symmetrical balance, where both sides of an artwork are visually equal. Things on either side of a center line are almost alike.
Take this artwork for example. If you divide it vertically, then you have a symmetrical balance.
Here is another example of symmetrical balance:
Asymmetrical balance is when a larger shape is balanced by a smaller shape. You will see this type of balance in a lot of landscapes.
For most artworks, a balance artwork is centered on the natural axis, or center vertical and horizontal line. Many artists place the focal point in the direct center. This is called a formal balance.