Bumps, or Coils, what's the difference?
Bumps are a woolen prep. The fleece goes through a carder which brushes the fibers, but doesn't align them. The fiber comes off of the final roller in a sheet, is fed through a funnel to create a strip, and is then wrapped around a cylinder under a slight bit of tension. The bump is then pulled off the end of the cylinder, leaving you with the choice to either pull from the center or from the outside. The roving is light and thin, and does not need to be stripped further in order to spin a fine yarn. The bump itself is very portable - no piles of roving to come loose, drag, or tangle. If you spin this roving long-draw, you can create a truly woolen yarn which is lofty and light.
Coils are a semi-worsted prep. The carder that creates these has a final stage which further aligns the fibers, similar to combing although short fibers do not fall out. This carder also creates a slightly thicker roving, which then is fed to a pin-drafting machine which further attenuates & aligns the fibers. The final roving falls into a rotating tube which creates the coils. The pin-drafted coils are dreamy to spin from - they feed effortlessly and beautifully into your hands, and likewise do not need to be further stripped or prepped in order to spin a fine yarn. You can still create very light and airy yarns with this roving, or you can create very smooth worsted yarns.