Takoyaki is often translated as "octopus balls" in English, but that carries the connotation of octopus nards, which this is definitely not. Rather, it is pieces of battered octopus tentacles fried like donut holes and then covered with a sauce and mayonnaise—among other toppings.
For some, the idea of eating octopus tentacles does not sound appetizing. You can substitute it with sliced sausage, cheese, or mochi (rice cakes). That's totally fine.
Making takoyaki is a snap if you have a takoyaki hot plate (pictured). The takoyaki mix is sold at the supermarket, so all you need really are eggs, water, whatever you're putting in it, tenkasu, sauce, and mayo. The trick is turning the takoyaki so you don't burn it, while keeping the inside hot and gooey. I use an icepick to turn the takoyaki—flicking it. Some people use chopsticks or toothpicks.
Takoyaki is one of Osaka's famous "soul food" dishes. In other parts of the country, it's apparently viewed as more of a snack—like French fries or potato chips. In Kansai, it's a meal. Okonomiyaki is also a famous dish in Osaka. Here's a fantastic recipe for that.