What is the difference between `using` and `import` in Julia when building a module?

Question: What is the difference between using and import in Julia when I'm building my own module?

My guess based on reading the docs: using is used to bring another module into the name-space of the current module. import is used to bring specific types/functions/variables from other modules into the name-space of the current module.

So, how wrong am I?

The Julia Modules documentation states:

The import keyword [...] only operates on a single name at a time. It does not add modules to be searched the way using does. import also differs from using in that functions must be imported using import to be extended with new methods. [...] Functions whose names are only visible via using cannot be extended.

(Emphasis mine.)

For example, you can use import to add methods to Base.show to pretty-print your own types, but not with using.

There is also importall that imports all exported names of a module.

(This answer refers to Julia 0.6; the documentation was reworded for 1.0.)