Question

What is the main difference between clayey and loamy soil?

Answer 1

Clay is made up of the tiniest of tiny particles of ground up rock. It is frequently dark orange or red due to the oxidized iron (rust) it contains.

Loam is a mixture of different size particles. The ideal mix is 40% sand (large particles of rock), 40% silt (medium to small particles of rock), and 20% clay. Since loam is made up of rock ground to varying degrees, it is inorganic, however, since it is natural and contains no synthetic ingredients, plants grown in loam can still be considered “organic”.

Mix 50% loam with 50% well rotted compost and you have the ideal planting medium for most terrestrial plants.

Answer 2

Clay has super fine particles that cling together and prohibit water and nutrient movement, while sand has course particles which allow water and nutrients to leach too rapidly. 
There actually is one more classification called silt which has particles sized between clay and sand.
Loam is a mixture of these sizes and is the favorite of most plants because it is usually richer in nutrients and humus and will retain water while allowing the excess to drain away. 
There are other soil types in between these as well, sandy loam, clay loam, silty clay, etc.