Tako is octopus and is a very common ingredient in Japanese restaurants.
The texture of octopus is almost rubbery, but when prepared well, it can be more tender. This means massaging and boiling the octopus first.
Tako nigiri and sashimi is usually cut very thin to make it easier to eat.
The flavor of tako is mild and slightly sweet. Other than the texture, it is a great beginner item for sushi neophytes.