Soil is made of tiny pieces of rock mixed with humus (decayed plant and animal material), air and water. The size of the rock pieces gives each soil different properties.
Sandy soil has large grains with big gaps between them. It feels gritty, and water drains through it quickly, so it dries out fast and loses nutrients.
Clay soil has very tiny particles packed tightly together. It feels smooth and sticky when wet and holds lots of water, but drains slowly and can become waterlogged. It can set hard when dry.
Loam is a mix of sand, silt and clay with lots of humus. It holds enough water and nutrients for plants but still lets extra water drain away and air reach the roots. Most garden plants grow best in loam.