Several different grounding electrodes can be used. It might be a literal rod buried in the ground, a group of 3 rods connected by copper wire, the cold water pipe, the steel structure connected to the concrete encased rebar in the foundation, if available, or a loop of wire buried in the ground surrounding the building.

They do get corroded and are supposed to be replaced every so often. Different materials are used depending on the type of soil to minimize the corrosion. If they're really fancy they might have a cathodic protection system to prevent corrosion.