A Single-Document Interface (SDI) is a type of GUI. Any program not capable of displaying more than one document per window is considered to use an SDI interface. Examples of this include Windows Notepad, IE 6 or earlier, and the windows command prompt.
The advantages of an SDI interface over a Multiple-Document interface (MDI) or a Tabbed document interface revolve mostly around simplicity. SDIs are far easier to program, and if implemented appropriately, easier to use as well. However, using an SDI limits a program's multitasking capability. If an operating system's built-in window management capabilities are insufficient, trying to use too many instances of an SDI program may result in the user frequently having to reorganize or search through the windows on their desktop. Additionally, too many individual instances of a program may strain the user's hardware. These problems were what led most modern web browsers to use tabs.
Usually, a program will only be referred to as using an SDI if it is being contrasted to a tabbed interface or MDI.