Aeration 

Aerating a lawn (also called core aeration) is one of the most beneficial cultivation techniques.  Cylindrical "plugs" of soil are removed from the ground, creating a grid-like pattern of small holes in your lawn.  The holes created through aeration allow the grass roots to have better access to water, nutrients, and air.  Over the next few months, the holes fill in with organic matter, the plugs dissolve, and the grass roots are strengthened.  The result is less soil compaction and a thicker, healthier lawn.

Scarification

Lawns create the thatch layer through the normal living growing and dying processes. The thatch layer is unable to break down itself. The material has the wrong carbon to nitrogen ratio (Too much carbon not enough nitrogen) much like the principle that allows the other "thatch" on the roofs of houses to last 25 years. Thatch layers below 0.5" can benefit from lighter treatments, anything deeper than this you will need to work hard on.