Clay soils are possibly the most difficult to improve, but do have a real advantage over sandy soils - so don't start feeling sorry for yourself just yet.
They are sticky when wet, and form hard lumps, which are impossible to break down when dry. In hot weather they form large, deep, cracks. These cracks can rupture roots, and cause moisture loss - which makes the problem even worse. Regular hoeing helps to fill the cracks and forms a surface mulch, which will help retain the soil moisture.
Without doubt, all clay soils can be improved quite substantially, with the addition of mulches of organic matter, by way of straw stable manure, or a good grade of peat. Composted bark - not bark chippings - is also good. A few years of such applications can provide you with a workable soil. Clay still - but workable.
If your clay soil is seriously waterlogged, then you will need to think (do something) about land drainage. Plants will not grow in waterlogged conditions
Now for the advantage. Clay soils are usually rich in plant nutrients. They also retain much of the fertilizers that you apply. This is because the soil moisture - which holds the nutrients to a degree - does not soak away, taking the nutrients with it. It may evaporate, but the nutrients stay put!
Loam Soils have all the advantages of both the above soils, with none of the disadvantages. Simple as that!
Most plants grow well in them, they are easy to 'work' in most weather conditions, and they hold soil moisture and therefore the nutrients quite well. As with both the above soil types, Added organic matter can only improve it more!