Okonomiyaki is a Japanese pancake-like food where many vegetables, fish or meat are mixed. While described as pancake-like, it's not a dessert and is eaten for dinner or snack. There is not an "official" type of okonomiyaki—the word “Okonomi yaki” speaks for itself, since “okonomi” means “whatever you like” and “yaki” means “grilled” so in other words, it's simply grilled pancake with whatever you like.
One thing most okonomiyaki have in common is the sauce. Usually most okonomiyaki are eaten with mayonnaise and tonkatsu or okonomiyaki sauce, with some shredded red ginger and aonori (dried seaweed flakes).
Okonomiyaki also differ by areas in Japan. The two major area-based okonomiyaki are the Kansai area version and the Hiroshima area version.
Kansai okonomiyaki is a traditional pancake type where you mix everything in one bowl and grill it at once. The batter is made of flour, grated yam, dashi (broth), eggs, shredded cabbage, and mixed-in ingredients usually contain green onions (chives), meats, octopus, squid, or dried shrimp. In recent years there are more modern types of okonomiyaki which include kimchi, cheese or even mochi inside. Okonomiyaki is quite a symbolic meal in the Kansai-Osaka area and could even easily be called the “soul food” of Osaka.
Hiroshima's okonomiyaki has just about the same ingredients as the Kansai version, but instead of mixing everything together, they layer it. They first grill the batter, then the cabbage (3 to 4 times the amount of the Kansai version), then the pork or whatever other ingredients you favor. Then the grilled ingredients are wrapped with noodles and eggs like a Western omelet. The order of cooking might slightly differ by area, but Hiroshima people believe that this is the true way to make okonomiyaki.