Both are liquid cooling — but one puts a cold plate on the chip while air still cools everything else, and the other submerges the entire server, eliminating fans and air handling altogether.
Once a facility commits to liquid cooling instead of air, the next decision is how much of the server actually touches liquid. Direct-to-chip (D2C) cooling, sometimes called cold-plate cooling, routes coolant through a metal cold plate clamped directly onto the highest-power components — GPUs, CPUs — while lower-power parts (memory, NICs, VRMs, storage) are still cooled by airflow across the board. Immersion cooling instead submerges the entire server, board and all, in a dielectric fluid that is itself either pumped through a heat exchanger (single-phase) or allowed to boil and recondense (two-phase), removing heat from every component the fluid touches with no reliance on air at all. The two approaches sit at different points on a spectrum from "hybrid liquid + air" to "liquid does everything," and that spectrum position drives very different retrofit paths, serviceability, and facility infrastructure requirements.