It's a combination of sand, silt, clay, minerals and organic matter that also contains some air and water. Clay soils are sometimes referred to as heavy soils and sandy soils are called light. As far as the gardener is concerned, clay soil can cause a number of problems. On the positive side, clay isn't necesarily all bad. It has good moisture and nutrition retention.
Clay soil drains slowly.
So, as snow melts and rains set in, clay soil tends to remain saturated long after average or sandy soils have drained. It is also much slower to warm up in spring. Clay compacts easily making it hard for the roots to penetrate, resulting in stunted root systems. With clay soil, there are often problems with frost heave and root damage as the soil freezes and thaws during changing weather. And it can vary in different parts of your yard.