Ganache vs mousse

Ganache is an emulsion; mousse is a foam. One is dense by design and the other holds air, and they fail in completely different ways.

Side by side

	Ganache	Mousse	Why it matters
Structure	An emulsion of fat and water, dense throughout.	A foam — air held in a chocolate-set matrix.	This determines every technique difference. Ganache is stirred to avoid air; mousse is folded to preserve it.
Characteristic failure	Splitting, when the emulsion breaks and the fat separates.	Collapse, when the air is knocked out or the foam was never stable.	A split ganache can be rescued by re-emulsifying. A collapsed mousse cannot be re-aerated.
Chocolate temperature at combining	Warm throughout.	Warm but not hot — around 45°C. Hot chocolate meeting cold cream seizes into lumps that will not fold out.	The seized-lump failure is the most common mousse mistake and it is entirely a temperature-mismatch problem.
Uses	Fillings, glazes, truffle centres, tart fillings.	A dessert in its own right, or an aerated layer in a composed cake.

When you need something dense that sets and can be piped, cut or coated.

When you want lightness, and where the dessert is served in the vessel it set in.

Making ganacheSet against Mousse, with the practical consequence of each difference.
Chocolate mousseSet against Ganache, with the practical consequence of each difference.

ChocolateHQ editorial synthesis — ChocolateHQ(citation identity confirmed; passage not re-read)
The Science of Chocolate — Royal Society of Chemistry(citation identity confirmed; passage not re-read)