Generally speaking, egg whites are whisked, cream is whipped, but the process is the same. The idea is to get as much air into the whites or cream as possible by beating very vigorously, so that they puff up in volume, stiffen and hold their shape. You can use a fork, but it’s hard work. A wire balloon whisk or a rotary beater is a much better option, and will give greater volume in the end.
Whip double or whipping cream (but not single) until it just holds its shape, but don’t overdo it or you’ll end up with butter. Stiffly whisked egg whites will remain in stiff peaks when the whisk is pulled up out of the bowl. When a recipe tells you to whisk to soft peaks, the tips of the peaks should flop over when the whisk is lifted out. Always use whisked egg whites straight away before they collapse to a puddle.