Starting with seashells
Limestone is a kind of sedimentary stone that is very common all over the world. This kind of stone forms out of the shells of zillions of little tiny sea snails and creatures like that. These snails and stuff lived in the sea, billions of years ago. When they died, they fell to the bottom of the sea and rotted. But their shells were made of calcium like your teeth. So the shells did not rot and just stayed there. Pressure from other shells, and from the water, and from sand being washed over the shells, squashed the empty seashells all together into rock.
Many many years later, the sea changed where it was, and that left all this calcium-rock (limestone) on the land where people could quarry it (dig it up).
Travertine and marble
Limestone is also the main way you get lime, and lime is one of the things you make cement out of. To get lime, you have to burn limestone in lime kilns. What is left after you burn limestone is lime. In the Middle Ages, people in Europe and the Islamic Empire burned most of the statues of ancient Greece and Rome to turn them into lime. They used the lime to make mortar and cement, and built new buildings out of that.
Learn by doing: find a building made out of limestone near you.
More about sedimentary rocks
More about travertine
And more about marble
DK Eyewitness Guides: Building, by Philip Wilkinson (2000).
Geology: A Golden Guide from St. Martin’s Press, by Frank Rhodes (2001).