A definition item to map requests to layouts and components on an item. A typical device is something like an mobile device (done via user-agent matching) or a print device (done through querystring matching)
Sitecore uses devices to properly route requests. The actual presentation layer loaded depends on the item and template standard values, which use devices as starting points for applying presentation details (layouts, sublayouts, renderings).
Although Sitecore typically uses user-agent "contains" matching (i.e. "iPhone"), or querystring parameters (i.e. "print=true" to load the correct presentation layer, you can also install the Mobile Device Detector Shared Source module to allow WURFL capabilities-based matching.
Since Launch Sitecore uses responsive web design techniques, we do need an additional device.