A tree is a tall woody plant. Plants are all sedentary lifeforms feeding off sunlight, air, and water. Trees are the largest of plants, and the tallest. They reach highest (except for whisps and baubles, which never developed on earth anyhow), so they have unobstructed sunlight.
Achieving such height requires wooden trunks and branches. Wood is almost an archetypal structural material: stiff, light, flexible, a fractal biologically constructed nanocomposite, capable of supporting great weight. And trees have great weight, yes they do. Wooden roots anchor trees to the ground, supporting typically one large cylindrical trunk reaching up towards the sky, with wooden branches splitting off in all directions. All the parts are simply connected, with just one path connecting any part to any other part. At a height, where the sunlight is unobstructed, the branches split into a plethora of green leaves to capture the air and sunlight to make food. Ah, but they need water too. The tree contrives to pump water from the ground up through the wood of the trunk and the branches to the leaves. Trees are very stingy with their water, they need surprisingly little each day relative to their size.
Placing so much mass so high has a downside: trees have to withstand a lot of sideways force from the wind. The trunks and branches are fairly compact, but the leaves of trees have a huge surface area. Rather than leaves being huge sheets, they tend to be small, only a few centimeters across, and attached flexibly. Wind batters the leaves about, but the leaves don't cup to catch the wind, so wind blows through the trees. This has the upside of shaking dust off the leaves so they can more effectively gather sunlight. Leaves put up with a lot of abuse, so they cannot be very fragile. The sound of leaves flapping and rubbing against each other in the wind is called a "rustling" sound. Some trees are very dynamic in the wind, even though they do not move on their own and are rooted to the ground in a fixed place. No earth trees have developed so far to feed off the mechanical energy of the wind moving the leaves and branches, not even in very windy places. When many trees grow all together, in a "forest", no one tree has to bear the full brunt of the wind.
Earth has birds and insects and mammals that trees have to guard against. Insects are small flying creatures that can reach anywhere. Mammals are usually ground-dwelling, so they can't reach heights. Birds are larger flying creatures, which usually eat insects. Also fire: trees are flammable. And in some places, winters.
Cylindrical trunks have a thin living layer (the "cambium") they need to protect, which slowly grows outward over time. This layer slowly builds wood inside, forming the structural support of the trunk, and bark outside, for protection. Trees vary in whether the bark is fractured, smooth, striated or irregular, thin or thick. The bark has to protect the trunk and branches from insects mammals and fire.
Leaves are almost all living. They are protected from small fires by being living, moist with water. Being at a height protects leaves from mammals but not insects. There is an ongoing battle between trees and insects, where trees make their leaves poisonous and insects learn to be immune to those poisons. Near the poles, winters turn water into ice, and snow covers trees, and sunlight is limited, so the effort of supporting living leaves is less worthwhile. Trees vary on whether the leaves just tough out the winter, or if the tree lets the leaves die, retreating into just its branches and trunk and roots and living off its reserves until winter passes. Dropping leaves continuously build up soil. Trees can control what other plants grow in their immediate environment by how their dead leaves decay, and also directly by their roots.
Trees, being living things, do need to reproduce. Most produce seeds: very compact capsules capable of growing a new tree from scratch. Seeds are often under a centimeter across, which is impressive compared to the immense size that trees can grow to. Seeds contain full instructions for a new tree, and enough food to bootstrap a new tree until the first leaves start feeding it. A new tree needs access to sunlight and water. Trees are high-R creatures: they produce many times more seeds than successful offspring. It's a given that most seeds will fall someplace without enough sunlight or water. In fact, seeds that fall straight down are almost guaranteed not to have enough sunlight, because the parent tree is taking the sunlight. In forests, often the only way for a new tree to be successful is for an old tree to fall, leaving a hole for sunlight to break through. New trees that happen to have enough water and sunlight grow very quickly, racing to take advantage of their niche and fill it before any other new tree can. Trees can live a long time, sometimes thousands of years.
Many trees co-opt the insects, mammals, and birds to reproduce better. They design seeds with a surrounding fruit that is tempting to eat, animals eat the fruit and the seeds, and the animals later expel the seeds far from the original tree. Some trees prefer whisplike seeds that rely on the wind to carry them elsewhere. Some trees co-opt insects for sex as well: they have flowers with liquid nectar for insects to drink, and make sure a powder of pollen (the male part) is rubbed on the insects' fur when they drink the nectar. When the insect goes to a flower in the next tree, the pollen gets rubbed off on the stamen (female part), and voila sex is achieved across large distances. Some trees develop quite elaborate flowers and fruits.
Some areas have a shortage of water. Here trees grow in gnarled globes, with thick leaves and prickers to ward off thirsty mammals. Some areas have plenty of water, then trees compete for sunlight, with tall trunks and branches all reaching up competing for height. Some places, particularly Africa, have little water but also very hungry mammals, then they grow straight up to just the height that the tallest mammals can't reach, then they branch out into a large flat disk of thick leaves.
Being a tree is a strategic lifestyle, not a tactical one. Getting a tree started in the first place is tricky, but trees leave that to chance, producing enough seeds so that they get lucky often enough. Trees find a spot with water and sunlight, and they sit there and grow. No thinking required, really. They just sit in the sunlight, and grow, and rustle in the wind.
This was in response to a prompt on reddit.com r/WritingPrompts, "Describe a tree."