Dendrobium is a massive genus containing more than (conservatively) 900 species. These range from cool-growing miniatures to huge specimen plants growing in hot conditions year round. The most commonly found types of Dendrobium are the phalaenopsis type (commonly known as den-phals, and named for the species most used in their breeding, which used to be known as Dendrobium phalaenopsis, but which is now known as Dendrobium bigibbum), and the nobile type (named for the species most used in their breeding, which is known as Dendrobium nobile).
A very showy flower, this type of orchid comes in colors that include white, yellow, brown, purple, red, and a few varieties that are multicolored. They can be a challenge to grow, but they grow well if the temperature is below 80 degrees Fahrenheit because they are used to cool or cold climates. The Odontoglossum orchid has stems that can grow up to four feet high and blossoms that can reach up to six inches in width. Between 20 to 150 blossoms grow on each stem, and their petals are ruffled and very fragrant.
Always use a water breaker (a water diffuser that you attach to the front of your hose to soften the flow of water): For only a few orchids, a sprinkling can with a long spout with a rose (a water diffuser placed on the end of the water-can spout) that has many small holes works well. These devices allow thorough watering without washing out the potting material.
Cymbidium carry tall stems with many flowers that have a more typical orchid shape. They are easily identified by their grassy leaves, and they readily form large clumps. Most common are the cool-flowering types, which need to experience a cooler winter (nights in the 40s for some time in the fall) in order to initiate flowering. Once the flowers open, they can last for up to two months or more, especially in a cool environment. These are very showy and rewarding plants, and a plant in a six-inch pot can carry three stems with 15 or more flowers each, depending on the type.