Limestone is made out of remains of tiny shells and micro skeletons that are deposited on the sea bed. These include skeletal remains of aquatic organisms such as corals and sea snails. The pressure from the water, shells and sand compressed them together into the rock.

Coquina: Limestone that contains large pieces of shells or coral.
Chalk: Limestone that is formed from microscopic marine organisms.
Travertine: Limestone formed around a spring or a waterfall.
Oolite: Limestone that is formed around high-temperature areas such as tropical seas or lagoons.