The two methods
An infusion is water poured over material and left to stand. A decoction is material simmered in water over heat, usually for ten minutes or more. The difference is not intensity — it is whether energy keeps going in while the extraction happens.
The rule
If you can crumble it between your fingers, infuse it. If it is woody, a hard seed, a piece of bark or a root, simmer it. Nearly every disappointing cup in this category is a decoction that was steeped.
Where the rule does not hold
Three exceptions worth knowing. Marshmallow root wants COLD infusion, because its mucilage extracts better cold than hot. Schisandra wants twelve hours in cold water, because hot water pulls a harsh bitterness that cold does not. Sumac is steeped cold for the same reason. In all three cases the tradition specifies cold and the reason is chemistry rather than climate.
Roasted material is a third case
Roasted grains and roasted roots are already cooked, so they neither need simmering nor punish a long steep. The failure with these is under-dosing rather than under-extracting: two to four grams per hundred millilitres, which is roughly twice a leaf infusion.
Why a mixed blend is a compromise
A blend containing a flower and a root cannot be brewed correctly by any single instruction, because the two want opposite treatments. Commercial blends resolve this by cutting the root very small. At home, blending within a preparation style avoids the problem entirely.