Galvanic isolation

[1][2] Energy or information can still be exchanged between the sections by other means, such as capacitive, inductive, radiative, optical, acoustic, or mechanical coupling.

Optical isolation is generally very limited in power capacity, but it can carry very high speed data signals.

[4] Where capacitors are used for isolation from power supply circuits, they may carry special ratings to indicate they cannot fail in a short-circuit, possibly connecting a device to high voltage or presenting an electrical shock hazard.

Unlike opto-isolators they do not contain a light source with a finite life, and in contrast to a transformer based approach they do not require DC balancing.

In a relay, the control signal energizes a coil that produces a magnetic field that pulls on an electrically isolated armature with switching contacts.

Optocouplers are used within a system to decouple a function block from another connected to the power grid or other high voltage, for safety and equipment protection.

Power isolation transformers increase the safety of a device so that a person touching a live portion of the circuit will not have current flow through them to earth.

A transformer is the most widespread example of galvanic isolation.
An opto-isolator is a very popular method of isolation in digital circuits.
Cross-section of dual in-line package opto-isolators. Relative sizes of LED (red) and sensor (green) are exaggerated.
Everlight EL817 phototransistor opto-isolator in DIP-4 package
Halo TG110-S050N2RL 10/100BASE-TX Ethernet pulse transformer in SO-16 package, with the underside shown on the right.