Sea urchins are herbivores.  They graze on broken off algae that have sunk to the bottom.  Urchins do have a food preference of Macrocystis pyrifera.  If many urchins are present they can even devour a kelp forest.  They have been known to eat barnacles too, because they will graze right over them. 

Some predators to the purple urchins are octopus, crabs, fish (Semicossyphus pulcher), starfish, seagulls and otters (where populated).  The red urchin has an even bigger predator, humans.  In Japan the red urchin is considered a delicacy.  They began collecting them comercially in the 1970's.  In Japan they can be as expensive as $6.50 per ounce.  It also takes about 20 fresh urchins to obtain 1 cup of "roe".   They eat the gonads, "roe", of the red urchin.  This is very expensive type of sushi.  There is a big commercial fishery for them and they are collected and exported.  In the absence of predators, urchins can do major damage to kelp forests.