Growth ring

A Growth ring is the ring of growth found in some living organisms, such as trees. Some animals such as shells and corals also have annual growth rings.
Counting the growth rings shows how old the organism is, or was when it stopped growing. For example, the growth rings in furniture, walls and other wooden items can be counted.
How tree rings are developed.
Tree rings grow under the bark, and the bark is pushed out while the tree is growing. The inner part of a growth ring is formed early in the growing season, when growth is fast and is known as early wood. The outer portion is the late wood, and is denser than early wood. Many trees in places with hot summers and cold winters make one growth ring a year.
For the entire life of a tree, a year by year record or ring pattern is formed that reveals the climate conditions in which the tree grew. If a tree grows in a dry place, you can tell its age better than when it grows in a wet place. Switching in between bad and good conditions can result in several rings forming in one year. Tree ring growth patterns are affected by the climate, weather, rain, temperature, soil pH, plant nutrition, carbon dioxide concentration, and sunspots.
Rings happen because of the change in growth speed through winter, spring, summer and fall, so one ring usually marks the passage of one year in the life of the tree. Tree rings are more visible in places where the seasons change between hot and cold. Enough moisture and a long growing season result in a wide ring. A dry year may result in a very narrow ring. Trees from the same area will tend to grow the same pattern of rings. 
Tree growth rings can tell us about the ancient climate and growth rings can be used to tell the age of a tree. Growth rings can also be used to date the wood in old buildings, ships and frames for paintings. Tree rings are used to make radiocarbon dating more accurate.
Some perennial herb species form annual growth rings in their roots that basically follow the same principles as growth rings in trees.