Wreak is almost always only heard in the popular idiom "to wreak havoc", which means to inflict devastation or damage to something or someone. You can also say "to wreak anger", which means to let loose your anger on somebody, e.g. "He wreaked his anger on the children". It's pronounced the same as the word reek (meaning to smell strongly).
Wreck is used far more often, usually related to traffic accidents, and simply means a thing that has been ruined or broken: a car wreck, building wreckage, shipwreck etc. It's pronounced as it looks, with a silent "w": rek.
For example, "the recent earthquake in Japan wreaked havoc and caused a great number of wrecked homes".
In the last week I've heard more than one person say "wreck havoc", which made me think that perhaps these words, though their meanings are somewhat related in meaning, are often confused with one another.