To dethatch or to aerate, that is the question.
Many people mistakenly use these terms interchangably or synonymously. They are in fact very different processes used for different problems.
Thatch is what we call the layer of dead grass parts just above the soil surface of a lawn. Some thatch is beneficial and neccessary. but sometimes it gets too thick and causes problems. Thatch build up usually results from feeding or watering too much or from mowing too infrequently and leaving thick piles of clippings on the lawn. Thick thatch can harbor insect pests and keep water and nutrients from penetrating down to the grass roots. A dethatcher has tines that dig down tothe soil and pull up the thatch. After dethatching often you can overseed the lawn as well.
In most climates, the action of frost in Winter will break up hard clumps. In some cases, such as with heavy clay soils, compaction may be a problem. If this is the case, aeration may be the answer.
An aerator rolls over the soil and punches core holes out of the ground. This will enable water and nutrients to better penetrate the soil.