"import foo" and "from foo import bar"

[Juanjo Álvarez]

I have a question regarding the difference between those two, apart from 
having to type the module name before the symbol in the
first case.

If I am only using one or two simbols of a module (example: string): 

It's better to write the "from string import join" form or it's exactly
the same (in memory use and performance) that "import string"?