Takoyaki is sold nationwide in Japan but it is definitely the soul food of Osaka. Tako means octopus in Japan and yaki means fried/grilled/baked. It is a ball shape topped with dried bonito flakes, special sweet sauce and dried seaweed powder. To make the takoyaki, you should have the special takoyaki grill plate with holes.
It is quite interesting that people in Tokyo do not have this grill plate and usually buy from takoyaki stands(it is quite rate to see on the menu of a restaurant) and eat on the street or take away to home. Then, it is considered as a snack, not a meal.
On the other hand, people of Osaka and its surrounding area own these grill plates at home and frequently cook takoyaki at home. Therefore people of Osaka consider takoyaki as a meal. Of course takoyaki stands are more located in Osaka area than in Tokyo, takoyaki attracts local people and tourists. Takoyaki stands can be seen at the festivals and temples or shrines when there are big events.
Please watch out when you eat the takoyaki. It is extremely hot! Outside of takoyaki balls are well fried but usually inside is more creamy and inside temperature is hotter than the outside temperature. However do not leave the takoyaki until it gets cool. The fresh hot takoyaki's taste can sometimes overwhelm the taste of sushi!