An informal leader is someone within an organization or work unit who, by virtue of how he or she is perceived by his peers (or others in the organization) is seens as worthy of paying attention to, or following. The major thing that distinguishes an informal leader from a formal one is that the informal leader does NOT hold a position of power or formal authority over those that choose to follow him or her.
The ability for an informal leader to influence or lead others rests on the ability of that person to evoke respect, confidence, and trust in others, and it is not uncommon for an informal leader to NOT intentionally try to lead.
Informal leaders can be exceedingly valuable to organizations, and to the success of formal leaders, OR, if informal leaders do not support the formal leaders and their agendas and vision, they can function as barriers in the organization.
For more on informal leaders:
Understanding Informal Leaders In Organizations (and Benefiting) by Robert Bacal
Developing The Informal Leaders In Your Organization by Robert Bacal
Benefiting From Informal Leaders In Your Organization by Robert Bacal