Insulation, simply put, is a quality of a substance to 'resist' transferring heat (or cold, basically energy) through it. In building construction high insulation is used to allow the internal environment temperature to maintain constant independently of external temperature changes. This not only makes the inside a more 'pleasant' place to live, it helps reduce heating and cooling costs by making it easier to keep at the same temperature; which in turn means your home is more ecologically friendly, as you are consuming less environmental resources.
Usually the R value is given for a certain type and thickness of material as installed (often known as the 'added R value'); i.e. a low density glasswool batt would need to be 130mm installed to achieve an R of 2.5, but only 100mm thick of medium density. Note: We say 'as installed', taking a low density batt that is designed to work in 130mm as installed and squashing it to fit in 100mm will not be the same as using a medium density batt in the first place.
All main building materials (be it wall, floor, ceiling, loft or roof components) have known R-Values. A table at the bottom of this article gives the R-values for many common materials. You can use this table to work out the 'ideal' R-values you want for your building in different seasons. Yes, the R-value of a material can vary depending on the 'mode' of heat transfer you are trying to block (radiant or conductive); so for different seasons it can be advantageous to use materials with different qualities to suit whether you want to stop heat getting out (Winter) or heat getting in (Summer).
Obviously the higher the R-value of a material the better an insulator it is, but this usually also implies higher costs. So there is also an economic driver to find the best suited R-value material to given situation. For instance ceiling insulation often comes in the form of insulation batts, this makes it relatively easier to handle and install compared to the older 'loose fill' variety of ceiling insulation. Its also somewhat safer for the installer as well.
Also of importance is the degree of external temperature range you need to deal with where you live. Basically its easier to suitably insulate a property in a mainly temperate climate than one in a desert or snow zone! You will often find your local building regulations have guidelines on what insulation is required in different climate zones.
R-Value Calculator - Work out the R-Value of a set of materials.