As far as horticulture (the art and practice of garden cultivation) is concerned, there is no exact definition of when a plant is a shrub or a bush. A good general description of a shrub is a woody plant with several perennial stems that may be erect or may lay close to the ground. It will usually have a height less than 13 feet and stems no more than about three inches in diameter.
Many people use both "shrubs" and "bushes" to describe their plants. What's the difference, then? Overall, it's really just a matter of personal preference and regional language.
In classic horticulture, a bush usually refers more to the shape something makes than the type of plant it is. For example, when describing a plant, you might say 'forms a bush' (as opposed to being tree-like or growing straight up). In classic horticulture, the shrub can specifically mean a plant that maintains its structure above the ground all year round. It cannot be split or divided because there is only one set of roots at the base of the entire plant. Shrubs can be evergreens, but they don't have to be and some shrubs may be considered small trees, but can still be defined as shrubs.
Boxwoods are evergreen shrubs that can be easily shaped and pruned—they're used to make the iconic Disney topiaries featured at their theme parks around the world.
Distinctions Based on Location
Another way to delineate between the two plants is to consider the setting of the plant in question. For instance, some gardeners think of specimens that are cultivated in a garden to be considered shrubs. Bushes, on the other hand, are those plants out in the wild that fit the definition of a shrub.
While this is a good suggestion, it may not always be a hard and fast rule. Consider both rose bushes and butterfly bushes. These are both often found to be cultivated in gardens and are not known as rose shrubs or butterfly shrubs, even though according to a location-based definition, they would be shrubs.
Others have different ideas about the difference between shrubs and bushes. The Oxford Dictionary says that a bush can be a shrub, or also a whole cluster (thicket) of shrubs or shrub-like trees. Others may say that a shrub is smaller and out in the wild, or similar variations.
A Difference in Foliage
Another popular way to distinguish between bushes and shrubs is through their foliage. Some consider a bush to have stems and leaves that are almost touching the ground. It can be found in the wild and may grow and intertwine with other bushes and wild plants or grasses. A shrub can be taller than a bush, but not as tall as a tree and have thicker foliage than a bush. A shrub can be groomed, pruned, and shaped while a bush is usually left to grow wild.
In the end, all you need to know is that there is no widely accepted difference between shrubs and bushes.