Triangle
A closed loop with three vertices forming a triangle. One vertex connects to an external line (typically a photon); the other two sit on a fermion line. Three internal propagators.
Structure
Three vertices connected in a triangle by three propagators. For the QED vertex correction: two fermion propagators and one photon propagator form the loop, with the external photon attached at the apex.
Where it appears
Vertex correction: gives the anomalous magnetic moment $a_e = \alpha/(2\pi)$ (the Schwinger result). The UV divergence is related to the self-energy divergence by the Ward-Takahashi identity.