When you ball up a piece of paper in your hand, you crinkle it — you make a smooth thing wrinkled.
Use the verb crinkle to describe what happens when you ball up a freshly ironed shirt and shove it in a suitcase, or the way your favorite teacher scrunches her eyes when she smiles. You can also call the creases or wrinkles themselves crinkles. You might become so annoyed at the crinkles in your curtains that you take them down and iron them. Crinkle comes from the Old English crincan, "to bend or to yield."