The teishoku, or set meal, is a staple of everyday Japanese dining. Nearly every restaurant (except most izakayas) will have a selection, and they're almost always a good deal.
A typical teishoku arrives as a main dish, a bowl of rice, miso soup, and a small dish of pickles. Sometimes everything comes together on a square tray; sometimes the courses arrive separately. The main dish depends on where you're eating — a katsu-ya serves various tonkatsu sets, an unagi-ya offers grilled eel, a tempura-ya does tempura, and so on.
For travelers, teishoku are ideal: you get a complete, balanced meal without needing to navigate a complicated menu. Look for the word 定食 on signs outside restaurants, often with plastic food models in the window showing exactly what you'll get. Lunch sets are particularly good value, often running 800–1,200 yen.