I've most often seen "systems software" used to designate software whose primary users are other programmers or systems as opposed to end users (e.g. databases,

I've most often seen "systems software" used to designate software whose primary users are other programmers or systems as opposed to end users (e.g. databases, message queues, monitoring tools, etc.). Ruby is typically used for writing business web applications and while those applications run on the back end, their primary users are business/end users. Of course, there doesn't seem to be a clear consensus on how those terms are used and the line is often blurry between the categories.