A team leader or team lead is someone (or in certain cases there may be multiple team leaders) who provides guidance, instruction, direction and leadership to a group of other individuals (the team) for the purpose of achieving a key result or group of aligned results. The team lead reports to a project manager (overseeing several teams). The team leader monitors the quantitative and qualitative result that is to be achieved. The leader works with the team membership.
The team membership may not directly report or answer to the team leader, (who is very often a senior member of the organization but may or may not be a manager) but would be expected to provide support to the team leader and other team members in achieving the team's goals.
A good team leader listens constructively to the membership and to the customer(s) of the results that the team is charged with delivering.
The responsibilities of a team leader vary greatly between organizations, but usually includes some responsibility for team building and ensuring teamwork. The term is used to emphasize the cooperative nature of a team, in contrast to a typical command structure, where the head of a team would be its "commander".