Quite a bit if you want to get down to the nitty gritty, but at a high level we (the dept I work for) define the difference as being onsite or offsite. Onsite (virt) is our own system, housed in our data centre, scalable at a seconds notice for high profile systems and projects. Offsite (cloud) is a backend owned and hosted by someone else that we utilise as needs be, where data has been flagged as being allowed offsite.