The balance of an image can be thought of as the distribution of visual weight within the piece. A piece of art can be structured so that the elements create a sense of balance and steadiness or can be unbalanced and create a feeling of tension.
The balance of a piece can be affected by the arrangement of the shapes within the piece as above or by utilizing other elements of composition such as the arrangement of colour within the piece as below.
The balanced images above were made to feel balanced by creating Symmetry. In other words both sides of the image are exactly the same. A sense of visual balance can also be achieved without creating a symmetrical image. This is known as an Asymmetrical balance and is usually a much more interesting (though not so easy) way of creating balance. Asymmetrical balance can be achieved by using a variety of techniques but relies on using the relatively different "visual weights" of different colours, tones and shapes etc. For example: in the image below, the black circle is smaller in size than the gray circle. However the greater contrast between the black circle and the white background gives the black circle more visual weight than the less contrasting gray circle. Asymmetrical balance in this case is achieved by using a larger object that "weighs" less visually and a smaller object that "weighs" more visually.