Inevitably, keys get dirty. Especially if you eat or drink at the keyboard- who doesn't?
As others have said, wiping with a damp cloth is enough most of the time. For very minor residue I use Windex Multisurface, which contains a dilute lactic acid that removes food residue. When that is not enough, I will take all of the keys off and clean in warm soapy water. At the same time I take the opportunity to clean under the keycaps.
