Limestone is a sedimentary rock formed from precipitation(evaporation of water leaving salts) of minerals or from dead matter such as sea shells or fish bones.
The latter,which has an organic origin is the most common. Limestone with a non-organic origin is called travertine
Limestone is composed of calcium carbonate (calcite) which react with acid water to give calcium bicarbonate which is then washed away in solution. Limestone, unlike other rocks such as granite or gabbro easily gets weathered.
This is mainly because of the presence of joints and bedding planes(joints running horizontally to the rock) which makes it easy for weathering agents such as acids to penetrate at ease.
In addition to being weak, limestone rocks are permeable and pervious also making them prone to weathering agents.
