Actually, that's not true. Orcas belong to the family Delphinidae, which includes the so-called "oceanic dolphins." Porpoises belong to a different family all together - the family Phocoenidae. A quick look at the dentition of an orca will confirm that it is NOT, in fact, a porpoise.
That doesn't contradict what I said in the slightest. Also, according to the Linnaean taxonomic system, "whales" do not constitute a family - they comprise an ORDER (Cetacea).