Shrub

A shrub or bush is a category of plants. A tree usually has one stem, which at some height has branches. A shrub can have multiple stems from the bottom up. Usually, bushes do not grow as tall as trees, very often they are less than 1–2 m tall.

Very many  plants can be either shrubs or trees, depending on the growing conditions. Small, low shrubs such as lavender, periwinkle and thyme are often termed subshrubs. Shrubs can be either deciduous or evergreen.

A shrubbery is a garden with shrubs as the main feature.

 Shrubs as a botanical structural form 
In botany and ecology a shrub is more specifically used to describe the particular physical structural or plant life-form of woody plants which are less than 8 meters high and usually have many stems arising at or near the base.

For example, a descriptive system widely used in Australia is based on structural characteristics based on life-form, plus the height and amount of foliage cover of the tallest layer or dominant species.Costermans, L. F. (1993) Native trees and shrubs of South-Eastern Australia. rev. ed. 

For shrubs 2–8 m high the following structural forms are categorized:
 dense foliage cover (70–100%) — closed-scrub
 mid-dense foliage cover (30–70%) — open-scrub
 sparse foliage cover (10–30%) — tall shrubland
 very sparse foliage cover (<10%) — tall open shrubland

For shrubs less than 2 m high the following structural forms are categorized:
 dense foliage cover (70–100%) — closed-heath
 mid-dense foliage cover (30–70%) — heath
 sparse foliage cover (10–30%) — low shrubland
 very sparse foliage cover (<10%) — low open shrubland

 References 

Other websites
Selecting Shrubs for Your Home (University of Illinois Extension)