A document is a unit of your stuff: a photo, a letter, a story, a song, or anything like that.
So a document is the same thing as a file?
Not quite. A file is a unit of any sort of information stored by a computer. A document is a file, but there are also other files that aren't documents.
For example, there'll be lots of files on your computer that aren't even designed to be read by a human—software is made up of files designed to be interpreted by the computer.
How about folders? Is a folder a document?
Nope—a folder (or directory) is a logical grouping of files, used to keep those files organised.
Note that I wrote “files”: any sort of file can be organised into a folder, not just your documents.
There will (most likely) be a folder somewhere on your computer designed to hold all of your documents, typically called something like your “Home” folder, “My Documents”, or “Documents”.
So a document is anything I’d want to print out and keep?
Almost—you can't print music or video (for example), but they're still documents. But, you wouldn't really want to print anything that wasn't a document.
So: a file is a chunk of information stored by a computer, and a document is any file that represents your stuff.