Limestone is a kind of sedimentary stone that is very common all over the world. This kind of stone is made 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, and when they died they fell to the bottom of the sea and rotted. But their shells, which were made of calcium like your teeth, 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 all this calcium-rock (limestone) was left on the land where people could quarry it (dig it up).
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
More about marble
DK Eyewitness Guides: Building, by Philip Wilkinson (2000).
Geology: A Golden Guide from St. Martin’s Press, by Frank Rhodes (2001).