The Collection 1 Quality Assessment (QA) Band being added to Landsat 4-5 and Landsat 7 data provides information and flags dropped pixels, radiometric saturation, clouds, cloud shadows, and snow/ice, as noted in the table below. This will allow the Landsat 4-5 and 7 QA band to become compatible with the Landsat 8 QA band. The Landsat 8 QA band will also be updated when the L8 Collection 1 reprocessing begins in 2017. The future Landsat MSS QA band will only include values for fill, non-cloudy, and cloudy pixels.
The Collection 1 Quality Assessment Band Web page provides more details on how the QA band is used for Collection 1 Level-1 data products.