A greenhouse is a structure with transparent walls designed to absorb and trap heat and infrared energy from the sun. This allows the interior to remain at a higher temperature than the outside air, allowing plants to be grown in conditions that might not otherwise be suitable for them. A greenhouse also tends to trap humidity in the warmer air, providing a better environment for plants in arid climates.
Passive & Active
A passive greenhouse relies completely on the sun for warmth, and must be carefully constructed and situated to absorb the proper amount of warmth and maintain the correct temperature. In many areas, natural sunlight is not enough to provide adequate heat, so supplemental heating systems (active heating) may be necessary to maintain growing temperatures. You can couple active heating with ventilation systems to provide a carefully-controlled growing environment suitable for almost any type of plant.
Extending the Season
Chief benefit of using a greenhouse is to allow you to extend your growing season. Many delicate plants only thrive during the spring and summer months, but the additional heat provided in a greenhouse may allow you to plant them several weeks or even months early, and cultivate plants later in the year than you could otherwise. In an active greenhouse or in mild climates, you may be able to grow plants year-round.
Another benefit a greenhouse offers is protection. Plants growing outdoors are subject to the whims of the weather, and a cold snap or sudden change in precipitation can stunt or kill your flowers or vegetables. Also, insects and larger animals can damage or destroy your plants, undoing weeks or months of gardening work in a moment. A greenhouse allows your plants some protection from these hazards, as well as allowing you to spend time gardening even in inclement weather.
A greenhouse is also an excellent place to raise young plants. Many sprouts and seedlings are delicate when first grown, and are extremely susceptible to changes in their environment and temperature. Starting plants in a greenhouse allows you to nurture them through the early stages of growth until they're hardy enough to be replanted outside.
Stress reliever - get some sunlight, add table and chairs, a magazine rack and enjoy an environment that will give you many hours/years of enjoyment and relaxation.