namespaces modules - What the difference between a namespace and a module in F#?

I've just started learning F# (with little prior experience with .NET) so forgive me for what is probably a very simple question: What the difference between a namespace and a module in F#?

Thanks

Dave

Edit: Thanks for the answer Brian. That's what I wanted to know. Just a clarification: can you also open a namespace as well (similar to C# using statement)?