IDL is:
``Command-line driven'' means that you use it by typing instructions at a prompt. ``Scientific graphics package'' means that its purpose in life is to turn your dull files full of data into exciting-looking, informative, illuminating pictures.
This has several advantages: you can put a ``stop'' in and look at all the variables. You can have a quick look and write a proper program if you need to do something more than once etc BUT it makes it slower to run than say C