Traditionally, when a home baker made bread she would save part of the dough, roll it flat and place it in the oven. Often a schiacciata (literally 'squashed') is something about the same as a focaccia. A proper one has a dimpled texture, made from the baker squashing the dough between her fingers, creating something halfway between bread and pizza.
In central Italy they might have only salt, olive oil and rosemary on top (here's a good recipe); in Sicily, they are topped with herbs, onions, olives, cracklings, or lard (strutto).
In Tuscany, they make a plain version and a schiacciata unta (with lardo) and schiacciata con ciccioli: two kinds, the schiaccia di Grosseto and the schiaccia pizzicata ('pinched') di Montiano are registered as PAT. Outside of Sicily and Tuscany, commercially sold schiacciata are usually thin and crispy.
Images by gnuf, ideericette, Massimo Telò, Creative Commons License, Tkarcher, Creative Commons License