Each tree begins life as a seed, bringing forth foliage, buds and fruit, growing taller and thicker as it ages, and lives for years, depending on its variety and ease of life. Climate, weather and care determine a tree's exact life cycle.
Once planted, a tree seed germinates into a sprout. If weather conditions allow the sprout to survive, it will grow into a seedling, at which point it will change from looking like a plant to developing a thin skin of bark.
Saplings are taller and more mature versions of seedlings. Garden centers typically sell trees at sapling size, as they are the ideal height and build for transplanting.
Saplings become mature trees that sprout branches, grow new foliage and develop fruit, depending on the variety. If the tree does not bear fruit, it might develop buds instead.
A mature tree's trunk grows thicker each year, developing one new layer of bark over top of the old one. According to North Carolina State University, the outer bark layer is dead and helps protect the inner layers.
Mature, dying trees are called snags. Dead trees can remain standing until rotted through or decomposed enough that they fall over.