The USGS has begun to reprocess all of the Landsat images in their archive. These images will be released as Collection 1 data and be designated as either Tier 1 or Tier 2. All Collection 1 data will share common radiometric and geometric parameters. In the future, if these need to be adjusted, then all archived images will be reprocessed and a new Collection will be released.
Tier 1 data have the highest radiometric and positional quality. They have precision terrain processing and have been inter-calibrated across the Landsat sensors. The USGS recommends using Tier 1 data for all future time-series analysis.
Tier 2 data are still very good images. They may have more cloud cover, affecting the radiometric calibration and obscuring ground control points within the scenes.
The USGS will also release new higher level Landsat data such as Surface Reflectance and several spectral indices. You can learn more about these products at: http://landsat.usgs.gov//landsat_cdr_ecv.php
Collection 1 data will become available starting in August 2016. The USGS will begin with the Landsat 4-5 TM and 7 ETM+ scenes, working backward through time. They plan to begin processing Landsat 8 data in November. You can learn more about this new process at the USGS Landsat Collection page.
Initially these Collection 1 data will only be available on the USGS Earth Explorer site at: http://earthexplorer.usgs.gov/. Click on the Data Sets tab then expand the Landsat Archive section to view the Collection data. You will also be able to slect the Pre-Collection data that we have been using for years. In the future the USGS will also make Collection data available through the GloVis site