Bubble
A single closed loop inserted into a propagator. Two vertices connected by two internal lines running in opposite directions, forming a loop. The simplest one-loop topology.
Structure
Two vertices on the propagator line, connected by two internal propagators that form a closed loop. For the photon self-energy, this is an electron-positron loop (vacuum polarization). The fermion loop contributes a factor of $(-1)$ and a trace over spinor indices.
Where it appears
Vacuum polarization (photon self-energy): an $e^+e^-$ loop in the photon propagator. Drives the running of $\alpha$ and the Uehling potential.