Fold

Technique:
Add the ingredient that you wish to fold over the top of the mixture you wish to fold into. Use a large spoon or spatula and slip it down to the bottom of your bowl. Next, make a scooping motion, overturning the ingredients onto themselves. Keep doing this while rotating the bowl until the two are sufficiently mixed together. Don't be over-zealous in your mixing or you will cause it to deflate.

Purpose:
Folding adds air into a mixture, you should always mix cake this way if you are mixing by hand. This technique is most often used when adding beaten egg whites or whipped cream (or any other light and fluffy mixture).