How to distinguish between isolation and shielding?

The typical signal isolation is the photoelectric isolation. Using PV isolation to separate the input of the single-chip device from the output of that. On the one hand, interference signals can not enter the single-chip microcomputer system, and on the other hand, the noise of single-chip system itself will not be a way to transfer as conduction. And the shielding is used to isolate the space radiation. As for the part with loud noise such as the switching power supply, you need to cover them up with a metal box, so it can reduce the interference to the single-chip microcomputer system brought by noise sources. As for the analog circuit which is afraid of interference particularly, for example, high-sensitive weak signal amplification circuit could be shielded .But the important point is that metal shielding itself must be connected with the real ground.