In a correctly built system the neutral and the grounding conductors are joined at exactly one point, and that point is the main service disconnect. Everywhere downstream of it they must stay completely separate. This single rule explains a surprising number of the problems found in older panels and in subpanels added by amateurs.
The reason for joining them at all is fault clearing. When a hot conductor touches a grounded metal case, the current needs a low-impedance path back to the source so that a very large current flows and the breaker opens immediately. The bond at the service is what completes that path.
The reason for joining them only once is that both conductors run alongside each other back to the panel. If they are also connected at a second point, current returning from normal loads will divide between them, and the grounding conductor starts carrying working current. That is not what it is for. It means metal enclosures, appliance cases, water pipes and anything else bonded to the system now have a small voltage on them under load, and if the neutral ever opens somewhere upstream, that small voltage becomes the full supply voltage.
In a subpanel the correct arrangement is four conductors from the main panel, two hots, a neutral and a separate grounding conductor. Inside the subpanel the neutral bar is isolated from the enclosure, usually by removing a bonding screw or strap, and the grounding conductors land on their own bar bolted to the metal.
The common error is a three-wire feed to a subpanel with the neutral bar left bonded, which was permitted in some older installations for detached buildings and is not permitted now.
Detached structures deserve extra attention because the ground rod at the outbuilding does not replace the grounding conductor from the house. Earth is a poor conductor and will not carry enough current to trip a breaker. The wire is what does that work.
A quick way to check an existing subpanel is to look at the bars. If neutrals and grounds are landed together on one bar bolted straight to the enclosure, and only three conductors arrive from the main panel, that panel is bonded where it should not be. Correcting it means adding an isolated bar and, in most cases, pulling a fourth conductor.
Background reading: full guide
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