Few groups of plants have attracted as much interest and intrigue as orchids. In truth they deserve such attention. The diversity within the orchid family is truly astounding - it is largest plant family with over 20,000 described species. The variety of habitats they live in, the variety of growth forms they take, not to mention flowers that are almost any shape and range in size from a large grain of sand to larger than a human hand - is beyond expectation. With such mind boggling variety, one could easily wonder what makes them all orchids.
Remarkably, the answer lies mostly in their flowers. Orchid flowers are some of the more curious looking in the plant kingdom. Two things that set them apart from other flowers immediately. First is a modified, often highly unusually shaped petal called the labellum, or simply the lip. The other is a structure known as the column, which is an elongate projection at the center of the flower that is a fusion of the male and female sexual parts common to any flower, the stamens and style, respectively. Pollen is produced at the tip of the column and the stigmatic surface is usually on the underside. The pollen grains are held together by a sticky substance to form pollina. When these make contact with the stigmatic surface pollination is initiated. The pollen grains grow tubes down the length of the column into the ovary (the swollen green base of the flower) that contain the eggs. Here the eggs are fertilized by the male gametes and seeds are formed, often in the hundreds, if not thousands. Orchid seeds are very small, almost dust-like.
A typical orchid flower is composed of 8 basic parts: the lip, the column, two petals, three sepals, and the ovary. Most orchid flowers have an odd orientation called resupination. The easiest way to explain this is that the lip is typically large and broad and serves as a landing platform for pollinators such as bees. To be a good platform it should be at a low position so that the pollinator's weight is supported by it. To have this position of the lip, orchid flowers evolved such that the ovary twists 180 degrees and thus leaving the lip at the bottom position. Most orchid flowers have this orientation. A relative few don't and they are called nonresupinate. Beyond the lip and column, the rest of the orchid flower is not so unusual. The two petals typically are held laterally while the three sepals are held in a triangular orientation - the one at the top is called the dorsal (upper) sepal and the lower sepals are on the left and right lower sides of the flower. Of course there is variation within this basic scheme, but this is a typical orchid flower.
In terms of growth habit, variation is extreme. Two basic types exist - monopodial and sympodial. Monopodial growth form is where the leading growing point (apical meristem) doesn't typically branch, but grows in one direction with the old stem dying off as the plant continues to elongate. Leaves of such orchids usually are in an opposite arrangement to each other, forming a fan-like appearance. "Baby" growths can be initiated anywhere along the stem where the leaf attaches to it (that point is called the axis). The roots and flower stalks grow out of these points as well. Sympodial growth form is when the plant grows from a thickened stem called a rhizome. New growths occur sequentially and are self limiting - they mature, flower, and stop growing. Subsequent new growths occur at the leading edge of the rhizome from meristematic tissue commonly called "eyes". Many sympodial orchids can grow into large clumps or even extensive mats of growths, but not all do. Roots always grow off the rhizome itself, but flower stalks can originate from the base of the new growth or anywhere along the growth itself. Many orchids of this type have thickened stems on the new growth called pseudobulbs which serve as reservoirs of water and nutrients. Monopodial types tend to grow upward while sympodial ones tend to grow horizontally. Of course, many exceptions exist though.
There are other differences as well, such as the root structure, seed structure and development, but such a discussion is beyond this introduction. The above characteristics are enough to distinguish most orchids from other plants. Since I have made a separate category for terrestrial orchids I will handle mostly tree growing (epiphytic) and rock growing (lithophytic) types here. Terrestrial orchids are no less orchids than these however, having the same flower parts, growth forms, and so on, but I have separated them since most are so different in cultivation needs.