Dari/Farsi question

I know that Dari and Farsi are both versions of Modern Persian -- how similar are they? Is it like the difference between say, American and Australian English, where a few terms might (OK, more than a few terms) might be confusing but the basics are the same or is it more like the relationship between, say French and Spanish?

BTW -- I really love lurking on this list. The discussions are fascinating, even though I can rarely add to them.
cj