Okonomiyaki is known as a kind of Japanese-style pancakes or pizza. As the name indicates (okonomi means favorites in Japanese), you can add desired ingredients in it. The most common ingredient is cabbage, and meat, seafood, corn, and bean sprouts are often used. It's typically served with okonomiyaki sauce, mayonnaise, and toppings, such as katsuobushi (dried bonito flakes), green onion, pickled red ginger, aonori (dried seaweed powder), tenkasu (tempura crumbs), and so on. Okonomiyaki can be fast food, and there are lots of food venders sell okonomiyaki in Japan.
In Japan, there are mainly two areas where okonomiyaki is very popular: Osaka and Hiroshima. Okonomiyaki is cooked differently in the two regions. In Hiroshima-style okonomiyaki, chopped cabbage is usually cooked on top of thinly spread batter. Also, yakisoba noodles are typically added. In Osaka-style okonomiyaki, chopped cabbage is mixed with the batter and cooked together. It might be easier for beginners to cook Osaka-style okonomiyaki than to cook Hiroshima-style okonomiyaki.