The Church fasts by degrees, and the calendar marks each day with its level. On the strict fast, the core of the great fasts and of most Wednesdays and Fridays, the faithful keep from meat and poultry, from fish with a backbone, from dairy and eggs, and from wine and oil. How ordinary Wednesdays and Fridays are kept outside the fasting seasons varies by place; ask your priest.
What remains is a long and generous list: bread and grains, rice and pasta made without egg, beans and lentils, vegetables and fruit, nuts, olives, honey, and shellfish. The old distinction is between animals with a backbone and those without, so in Greek and Antiochian practice shrimp, crab, octopus and squid are eaten on fast days; some traditions keep from them too. In older usage the oil set aside is olive oil in particular, and local custom decides how this is kept.
Lighter days relax the fast without ending it. Wine and oil return, usually on Saturdays and Sundays within a fasting season and on lesser feasts. On certain feasts fish is allowed too: in Great Lent on the Annunciation and Palm Sunday, and the Transfiguration in the Dormition Fast. In Cheesefare week before Lent the calendar sets aside only meat, while some traditions set aside fish as well. Ordinary fast-day meals are simple: lentil or bean soup, rice with vegetables, bread and olives.
None of this is a diet, and it is not kept for show. The sick, the expecting and nursing, the very young and the very old, and travellers keep a lightened fast, and the measure of each person's fast is received from their priest. What is spared from the table is meant for the poor, and the fast of the stomach means little without prayer and kindness.