I often see the terms "folder" and "directory" being used in user interfaces as well as in programming almost as synonyms.
Do I have to stick with one? or can I use both interchangeably? Do users prefer one of the two? or does it create confusion to use them as synonyms?
Note: there's the same problem in Italian and German as well.
Originally all directories were called 'directories' - which is still what most technical people will refer to them as. This is why it is still the most common term on Command Line Interfaces (CLI). So anywhere that you expect technical users, rather use 'directory', which is used almost exclusively in computer science.
'Folder' was a skeuomorphic attempt to let non technical users understand what a directory is. Most people were used to paper folders which held either pages or other folders, so it was easy for them to conceptualise what a folder did. This is generally a good term to use when dealing with non technical people, as most non-technical programs still use the term, that that is what they most likely have experience in.
The term directory is hardly ever used anymore and both Windows and OSX use the term folder. Although they can be used interchangeably since they mean the same thing to a user, they technically are not the same. See also: https://superuser.com/questions/169457/directory-vs-folder
Both are a metaphor. While on a filesystem level the term directory (a listing of information) makes a lot of sense, in a GUI the idea of a "folding cover or holder" is a closer match to its use and graphical representation.
In cases such as these it's important to look at the environment your application will be used. Usually a user will spend more time in the OS and all other applications taken together than in your specific application. If the OS always talks about folders, you shouldn't be talking about directories.
You should use either one or the other. In general a "thing" or concept in an application should always have the same name throughout your application. Eg. in social media the terms "message" and "post" could be seen as synonyms, users may call the post a message or the other way around, but the application itself should be clear about wether it's the one or the other.
My recommendation: Use folder in GUI context and directory in CLI context.
This dilemma is also present in Spanish.
"Folder" and "Directory" have long past their original, analog meanings these days in digital use. In my experience, "folder" conveys something that contains files or e-mails and thus it's a bit more "physical" than "directory", which conveys something that contains more abstract information, like a list of links.
That said, "Folder" is more familiar to non-tech-savvy users than "Directory". Also, it translates better to a tree-based hierarchical navigation. On the other hand, "directory" is a bit more formal and may fit better to power users. It's down to who you are speaking to and which kind of information this hierarchy contains.
