Display drivers are plugins that receive image data from the renderer and output it in some fashion. Generally, they will either send it to a file or display it immediately to the screen.
In terms of files, the two main drivers for production use in RIS write TIFF and OpenEXR images, respectively. Both of these drivers can create files with pixels ranging from 8-bit quantized all the way up to full floating point HDRI, and with any number of channels per pixel. Both drivers can write out periodic checkpoint images for incremental rendering, though currently PRMan can only restore from OpenEXR images.
For screen display, a good choice is the "it" driver for sending images to it3 for display if you have RenderMan installed. This flexible viewer lets you easily scroll, zoom, adjust exposure, and review previous renders among other things. The base RenderMan Pro Server also includes a simple "multires" driver that display images using OpenGL.