Many people get "folders"
and "files" on their computer confused.
A folder is like a physical cardboard manila letter-
or legal-size folder that you use to store your tax returns, invoices, receipts, phone bills, and so on.
A file is one of those documents, i.e., a letter, fax,
phone bill, invoice, receipt or whatever.
A file can be stored in a folder, but the reverse cannot happen: a folder
cannot be stored in a file. (Okay, so there's a way to
do this, but we won't go into it here.)