What is a tenor?
The tenor is the highest male voice type you will find in a typical choir.
This is the voice type with the smallest range, it barely covers two octaves. Despite this fact, tenors are the most sought after choir singers for 2 major reasons. The first reason is that there aren't as many men singing in choirs to begin with. The second reason is that most male voices, singers or not, are baritones.
In the opera, the primo uomo – literally translated as first man or more easily understood as leading man or main character – is most often a tenor. You'll know he is a tenor from the ringing quality which is very audible in this voice type.
Many a baritone will try to use this technique to classify as a tenor and some will be successful. These singers are easily visible by their red faces when trying to sing the high notes in the tenor melodic line.