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.)