While stiff (or firm) peaks stand straight up as soon as the mixer is lifted from the whipped cream, soft peaks will start to fall over just slightly, barely holding their shape. For whipped cream, you’re looking for soft peaks. Fun fact: if you whip your whipped cream way TOO much, it’ll go straight past stiff peaks to subtly sweetened (and vanilla-flavored) butter! Something to keep in mind: if you're making this whipped cream as a mousse (say, for some sort of mousse pie), you'll want to whip it to a stiff peak so that it's sturdier than a traditional, put-a-dollop-on-top whipped cream.