This is a Hawaiian dish which exists in a truly staggering number of variations, however it can be loosely described as a sashimi salad. Staple fish such as Ahi and Mahi-Mahi are common in poki, though pretty much any sashimi quality ingredient can be found in it. In terms of vegtables and seasonings, onion, sesame oil, ginger, seaweed, and shoyu are usually featured, however as with the fish, there are few limits on what any given chef might prefer to add in.
I can only give an anecdotal impression of this style of food, having only purchased it once. Being a big fan of sushi and sashimi I gave it a try at a fairly nice Hawaiian restaurant and thoroughly hated it. In that instance I found the fish and the vegetables to clash horribly with each other, creating a slimy morass of bad flavors. I again stress that this is only one impression of a food that takes many forms.