Rapid growth occurs during the spring and summer. In autumn, as the weather changes, growth slows. In winter, the tree is almost dormant. This seasonal change (in both weather and growth) can be seen in growth rings - alternating bands of light colored (and soft) with darker (and harder) wood.
Each pair (light and dark) represents 1 year's growth. The age of the tree can be found by counting the rings. Climate events such as forest fires, dry and wet years can be seen in unusual sized rings.
The living part of the tree(sapwood) is usually lighter in colour. The heartwood is older / denser wood that gives the tree most of it's strength.