As they sound, hop pellets are a variety of hops (Goldings, Fuggles, Challenger etc.) whose flowers have been finely chopped or mulched into a paste and then dried into tablets or pellets. Although this may sound like a waste of time some prefer them as they increase the amount of hop circulation in the copper and thus improve the level of extraction from them. The drawback in using these is that they may not be caught by the hopback in the copper and so enter the fermenter. Commercially hop pellets are quite commonly used along with the use of a whirlpool to separate the spent hop pellets and trub from the wort.