Plated Through Holes filled with conductive (or non-conductive) epoxy plugging ink, subsequently planarized and plated over. Via-in-Pads are usually patterned into a BGA pad for direct soldering. IPC-4671 contains various definitions of plugged vias.
In addition to to saving PCB real estate, Via-in-Pads are a conduit for heat transfers.
Voids / dimples, surface planarity and more.