bluefin, yellowfin, albacore, ahi,
There are many varieties of tuna, some as large as 1,500 pounds and others as small as 10 pounds, but all have in common a meaty texture (as opposed to flaky) and are moderate to high in fat. Bluefin is the largest tuna; adult bluefins have a deep red color and a luxurious texture especially prized in sashimi. Yellowfin (also called ahi) is also a high-quality tuna and have a pale pink flesh and a mild yet rich flavor. Albacore is the only tuna that has a nearly white flesh; it's the best of the tunas for canning. Small bonitos are the strongest-flavored tuna.
The best-tasting tuna (whether bluefin, yellowfin, or albacore) will range in color from deep red to pink. Ideally, tuna will be displayed as a whole loin, and steaks will be cut at your request. If already-cut pieces are all that's available, look for those that are moist (but not wet or weepy), and shiny. If they look dull and matte, or very brown, then they're probably old. Another sign of age is "gapping," when the meat of the muscle starts to separate into flakes. If you're in doubt about freshness, ask for a smell; the fish should have a fresh sea-air smell, not an overly fishy odor.
Be aware that there's actually no official definition or government enforcement for fish labeled as sushi- or sashimi-grade. Within the seafood industry, these terms are generally applied to fish that's very fresh or frozen, making it somewhat safer to eat raw compared with other, less fresh raw fish.
All tuna steaks will have a strip of darker meat running through them. This nutritious meat is perfectly edible but has a strong flavor that many people don't like. You can cut it out, or better yet, choose steaks with a minimal amount of it.
Keep fresh tuna refrigerated and use it soon.