The Consigliere is an advisor to the family and sometimes seen as the Boss's "right-hand man". They are used as a mediator of disputes, representatives or aides in meetings with other families. In practice the consigliere is normally the third ranking member of the administration of a family and was traditionally a senior member familiar with how the organization is run. They are also often the liaison between the Don and important figures, such as politicians or Judges. A Boss will often appoint someone close to him who they trust as their consigliere.
A consigliere's main job is to act as the Don's "auxiliary brain", helping the Don formulate plans. A consigliere, by the very nature of his job, cannot afford to simply be a yes-man to the Don, and is one of the few in the family who can argue with the Don on family matters. When the Don comes up with a business plan, the consigliere has to constantly challenge that plan's weaknesses until it's foolproof. The consigliere is the only one in a family who knows all of the Don's contacts and secrets, and if made to talk, could bring an entire family to ruin. However, there is neither a future nor potential for profit in this, and so long as he keeps the faith, he and his family will continue to collect his rather substantial wages whether he is imprisoned or dead.
Consiglieres are usually viewed as civilians or foreign diplomats, and are not generally targeted by rival families. It is almost an unspoken rule that a consigliere is a Sicilian, or at least an Italian, and always a family member or a trusted friend outside the Don's personal family. This rule was broken when Vito Corleone made the German-Irish Tom Hagen his full-time consigliere, causing a great controversy in the New York underworld.