Secondary fermentation is a misnomer. Fermentation occurs in the primary vessel. When fermentation is complete, one may optionally rack the beer to a secondary vessel to condition. Getting the fermented beer off the trub and flocculated yeast in the primary vessel helps it to condition more cleanly, and makes subsequent bottling easier.
There are certain flavorings in some recipes that are added when the beer goes to secondary. Hops are the most common addition at that stage. It's called "dry hopping", although that too is a bit of a misnomer, since the hops certainly don't remain dry.
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