The hardest part about growing plants is the lack of control you have over the most important factors for their development.   No matter how much time and effort you invest, there's simply nothing you can do for your garden if the weather is against you; your plants will overheat, freeze, or even drown and there's nothing you can do to prevent it.  In short, this is what greenhouses offer – control.  

A greenhouse offers the chance to take control of most important factors in gardening - the temperature and humidity of the environment in which your plants are growing.  The key principle behind their success is that glass has a different transparency towards light and heat.  Light like solar radiation is short-wave radiation, which easily passes through glass; heat like thermal radiation released by plants, is long-wave radiation, which does not pass through glass as easily.  Hence, although solar radiation can enter a greenhouse and heat up the plants, the thermal radiation released by the plants can't escape back through the glass, increasing the level of heat within the greenhouse.

The glass also provides another service for the greenhouse, as a barrier for the air.  When soil adsorbs solar radiation, it heats up – heating the air around it as well.  As the air close to the ground warms up, it expands and becomes less dense as the molecules become more active and start moving around.  The cooler air above the soil, however, maintains its density and, as the warm air rises, the less dense cool air falls to take its place next to the warm soil.  This cool air is then heated by the soil and similarly rises to be replaced by yet another new layer of cool air – so the cycle continues.  Eventually, even the first layer of warm air cools and descend to be reheated by the soil.  

In a greenhouse, however, the glass roof traps the warm air heated by the soil, so it cannot escape and be replaced by cool air.  Hence the air in the greenhouse continues to be heated by the soil, getting ever warmer throughout the day.  This warm air, together with the humidity from the water that evaporates from the soil, creates within the greenhouse an ideal environment for plants to grow.

When buying a greenhouse, however, it is important to consider a few extra features.  Firstly, there is not always a great deal of sunlight in this country, so it is wise to invest in a heating system to prevent the greenhouse from getting too cold.  Similarly, at other times there may be too much sunlight, so it is sensible to buy a greenhouse with some kind of ventilation system, for releasing air when the greenhouse becomes too warm, ensuring that the plants don't overheat.  Depending on how much time you have to dedicate to your plants, you can also purchase watering systems to regularly water the plants and maintain the humidity of the atmosphere.