Dolphins and porpoises are typically quite a bit smaller than the baleen whales, with the exception of the killer whale; killer whales are acutally the largest species of dolphin, and male killer whales can be almost as large as a minke whale.  Dolphins and porpoises are odonocetes (toothed cetaceans), and mostly prey on fishes and squid.  One easy way to tell if you are looking at a dolphin or a porpoise is by looking at the shape of the dorsal fin.  If it is triangular, it is a porpoise.  If it is curved, it’s a dolphin!  One offshore species, the northern right whale dolphin, doesn’t have a dorsal fin at all, making it very easy to identify.