队长 (duìzhǎng) is the noun for 'captain, team leader' — the designated leader of a team, squad, or unit in sports, work, or military contexts. 领队 (lǐngduì) overlaps as 'team leader' but is broader: it can refer to leading a tour group or expedition, and is also a verb; 队长 is more specifically the official head of a defined group. 队员 simply means 'team member', the opposite role.