What is the difference between topsoil and clay?

clays are derived from the minerals in rocks by chemical decomposition. They are the end product of weathering. There are different families of clays depending on chemical composition and crystal structure and there are various clays in each family. Soils are mixtures of rocks, minerals, and clays. Like clays, soils are grouped but by texture and mineral composition rather than by chemistory and crystallography. Given enough time a soil may break down to one or more clays but a pure clay will never break down to soil.

This was a very loose description of the differences. If you need more specific information you should be able to find it using one of the internet search engines.

The difference between topsoil and garden soil is an even finer distinction than the difference between soil and dirt, and it has to do with the amount of organic matter, and to a lesser extent, the types of trace minerals and the ratio of clay, sand and silt. Garden soil is a category of topsoil