In its broadest definition, a planter is a container for a plant. Some planters are stationary and may be built into the sides of buildings or other architectural features, but the majority are mobile.
The fact that the planter contains soil and water places certain constraints on its shape. For plants to be healthy, the water can’t just sit in the bottom of the container. It must go somewhere, and the challenge is to make sure that it only goes where you want. A planter must have an opening in the bottom for water to flow out, and if it is to be used indoors, some provision must be made for disposing of the water. For this reason, planters are generally used with some sort of tray or pan for catching the outflow, which may be attached to the planter or a separate piece.
A planter offers certain advantages, the most obvious of which is mobility. When plants are placed in the yard or a flower bed, moving them is a lot of trouble and generally not even contemplated. The mobile planter allows plants to be moved at the owner’s whim, perhaps switching them to different parts of the yard to catch maximum sun at all times of the year. In areas with harsh winters, planters allow owners to bring plants indoors.
Planters also function as a design element. They can have any color or design, and these can complement the shape and hue of the plants they contain. A planter can provide a splash of color or an interesting design, adding something new to a space.
Many planters are made of plastic, a material which addresses a couple of potential problems. Once again, the fact that the planter contains water presents issues. Metal planters may eventually corrode from the constant moisture, and wood can warp and finally rot. Plastic, however, presents none of these problems, and will remain attractive and usable for many years.