Trees are grown in a variety of forms (shape). Some forms are natural, but others are due to training and pruning over many years. This is why you will often see some varieties of trees in many different forms. For a more detailed explanation of these terms, see Explanation of Technical Terms
Generally, a tree with a single stem that you can walk under (usually a stem of about 61/2 ft) tall, expanding into a 'crown' above.
A tree with a shorter single trunk, which you cannot normally walk under, often grown for a small garden bed or container.
A tree that has a trunk less that 1m tall, usually grown for a decorative pot on a patio bed or raised planter.
A tree that is a single trunk, but has branches from the ground up, like a feather.
A tree that has multiple stems from near the ground.
A tree that has been trained onto a frame (usually a Bamboo frame) of varying proportions/sizes, pruned and tied onto the frame to fill it. A typical pleach will have a 2m trunk and a 2m x 2m frame on top to be used for aerial screening above a fence/wall or as a feature. However, many other forms are available pleached from the ground up to 2m or even 4m tall or on shortened trunks. Pleaching is often used to soften a wall or grow fruit trees along a wall and the bamboo canes can be taken off and replaced by a more permanent structure or by a stainless steel wire grid on a wall.