Balance is arranging elements so that no one part of a work overpowers, or seems heavier, than any other part. The elements may include the size, shape, colour, and arrangement of objects or shapes. When balanced, a composition appears stable and visually comfortable.
The three kinds of balance are symmetrical, asymmetrical and radial. However, balance doesn't necessarily mean symmetry. Asymmetrical ("informal") balance occurs when elements are placed unevenly in a piece, but work together to produce harmony overall.
Malevich, Black Square, 1913
Symmetrical compositions convey a sense of stability
Claude Monet, Impressionism Sunrise, 1872
Radial or rotational balance is based on a circle with its design extending from or focused upon its centre.
Asymmetrical compositions tend to convey a sense of movement because the elements of the composition are placed unevenly.
Vincent Van Gogh, Sunflowers, 1888.