There are a number of different degrees of roasting, the most common being:
Light – which gives a mild taste, popular in the USA
Medium – for a medium stronger well–rounded flavour
Full – produces a rich, full–bodied, well developed aroma and flavour
High – adds a strong roasty and bitter flavour, favoured by the French and Brazilians
Most well known brands are made from a combination (or blend) of coffees which come from different countries of origin and may contain different degrees of roast. These commercial blends are formulated to produce a consistent taste and aroma.
The purpose of blending is to combine a variety of coffee flavour profiles (i.e. the aromatics of one coffee with the acidity of other and the body of another coffee). For example, a Colombian coffee with high acidity may be combined with a Brazilian coffee with heavier body and no acidity. This will balance the flavour retaining a good body.