A Bonsai’s main elements:
The trunk, perhaps the most important element of a bonsai since it tells us more about the plant’s age than any other. It’s ruggedness should be allowed to develop naturally. Generally speaking, the greater the girth of the trunk, the older the tree.
It’s foliage, should have small leaves or needles. Examples of plants/trees with small leaves are Pomegranate, certain maples and oaks and most species of pine and spruce. The picture above clearly shows details of both trunk and foliage elements.
The roots, when exposed, lend age to the tree. Nursery plants most of the times have good looking roots. See example below.
In this picture, Bonsai creation by Bonsai sensei Masahiko Kimura one can admire the details of all three elements.