Decisions are most often taken by an arbitrator or leader or by majority vote. To oversimplify the issue somewhat, one of the expressed views prevails. It is commonly found in many of our institutions, such as courts of law, or in politics. In case you were wondering, I don't recommend it. In distinction to this, consensus does not pit one view against another.
Many group processes pursue consensus in a simple minded way.
In brief, the processes identify those issues on which there is already substantial agreement. If the different positions overlap a great deal then this is likely to be both satisfying and reasonably efficient in its use of time. On other occasions, however, there may be too little overlap. A decision which pleases some may grossly displease others. For this, more complex processes may be needed.