Core aerators are hollow-pronged tools that puncture holes in the turf and pull up plugs of soil leaving a hole about a half inch in diameter and 3 inches deep.
As your lawn ages the soil becomes compacted by various means of traffic. Compacted soil prohibits air circulation, water, and nutrients from reaching the roots. It also makes it difficult for beneficial microbial organisms to survive. The result is poor top growth and overall lawn health.
Aeration can be used as a preventative approach to controlling excess thatch build up. Thatch is, contrary to popular opinion, dead grass rhizomes which have a matting effect. Dead grass clippings collect on this "mat" and give the appearance that thatch is caused by grass clippings.
If your thatch is thicker than 1/4 an inch, aeration is an alternative for thatch removal that has much more beneficial results than raking alone.
If your soil is badly compacted a power aerator is your best option. If this hasn't been done for years you might consider this a priority in spring and fall for a couple of years. Once you have established proper lawn maintenance techniques then you can settle for a 'once every two to three year' aeration...in the fall.
Soil should be moist but not soaking wet. If your lawn is dry, water deeply a couple days prior to aerating.
Lawn should be cut a bit lower than normal before aerating. If you cut at 2 & 1/2 inches, cut it at 2 inches.
Aerate in at least two directions. Go north and south, then east and west. If you have the time and energy go diagonally, too. Your lawn will look like garbage, but this is only temporary. You will see a miraculous change in the health of your lawn after 3-4 weeks.
The soil plugs are left on the surface of the lawn to break down by the rain. Powered aerators can cover about 2000 square feet an hour or so and can be rented for home use. Hand aerators can be bought at a local garden supply store for around $30 and are only practical for areas the power aerator cannot reach.