asset.txt

rails assets:precompile -> This is the command you use to compress the assets .
  - This command will generate assets via using the asset pipelining. And have them
in public/assets folder.

rails assets:clean -> This will clean the older assets you have .
  - This command will clear older versions of the assets

rails assets:clobber -> Nuke public/assets .
  - delete everything.

For more information and reference : http://guides.rubyonrails.org/asset_pipeline.html