What is the difference between a CLS and non-CLS compliant object?

What is the difference between a CLS and non-CLS compliant object?
CLS Compliant "things" can be consumed by all .NET languages that are themselves CLS compliant--IronPython, Boo, etc
.Things that aren't CLS Compliant tend to be things that are only available to a certain language--
for instance, Boo's duck type is not CLS compliant and probably shouldn't be exposed in any public API,
because it only has meaning within the Boo language itself.