KiNgBrAdLeY7: Forget all the mumbo jumbo about cremes, yogurt, cucumber slices, etc. They will get your skin worse.
Depends on the severity of the burn.
For normal sunburn, quark works (even better than yogurt), I guarantee it. It not only reduces pain but also reduces inflammation, I've also used it for sports injuries several times. Not the low-fat type as that can dry out the skin further and the full-fat type could potentially irritate the burn so half-fat is ideal. Apply a non perfumed moisterizing cream afterwards but one that doesn't contain fat for aforementioned reasons.
Yogurt/quark is only a problem if the sunburn is very severe and there are open blisters on the skin, in that case do -not- use it as there could be an infection as a result (sort of common sense). If the sunburn isn't severe, quark and yogurt help quite a bit. Keep in mind that you don't keep that stuff on the skin forever, it's only for cooling for short periods of time.