Each time you upload a new list or add a contact to an existing list, you're also adding to the All Contacts list. We automatically do this for you to keep track of all the addresses you have in your account. If you've used VerticalResponse Classic you may remember it as the Master List. You'll see this as a list in the Contacts section with the name All Contacts.
This list works like any list in your account and can be used to send out emails; you'll see it as an option on the Send page.
It can't be deleted but you don't have to use it if you don't want to.