Balance refers to the distribution of optical weight in the layout.Optical Weight is the ability of an element (graphic text, headline, subhead, and so on) to attract the user's eye. Each element has optical weight as determined by its nature and size. The nature of an element refers to it's shape, color, brightness, and type. For example, a stunning color photograph of Mount Everest would have more weight than a block of text of an equal size. Following are some guidelines for understanding optical weight. 

http://faculty.rcc.edu/hewitt/Adobe_Photoshop/Balance/balance.html

Symmetrical Balance

Symmetrical Balance, the balance that achieves the object in image to be managed at the centre equally. This picture.

Asymmetrical Balance

The objects in this picture are completely separated by dissimilarity element. The objects' weight are clearly located to be distinguishing.

Radical Balance  

Asymmetrical at the black space and Radial Balance at the circle

The radical Balance in this picture is in a circle space which the balance focus at an object of the space inside.