I don't think there is much difference except that pellets are very finely chopped leaf hops and then compressed into pellets. The result is more of a muddy sludge in the bottom of your kettle, so no good as a filter bed. From a commercial point of view they take up less storage space and are easier to handle, weigh etc. They are very common in the USA, and that was all I could get from my local HBS when living in Florida. There I syphoned from the kettle after the hot break so was able to take off the wort above the trub leaving the hops and other trub behind rather than filter. The use of a sock on the racking cane also helped.