Like Chris said, phytoplankton are tiny plants; zooplankton are animals, often invertebrate larvae.
As far as feeding goes, one of the big differences is size. Phyto is much smaller. For clams, a mix of live phyto species, like DTs is a good choice. You can usually get that from the LFS, but be sure they're keeping it in the fridge. You can also grow your own, if you have the time to tend the cultures. There are also concentrated versions, like the stuff Reed Mariculture puts out, and it can be a great deal if you can find someone to split an order.