Aerating is the procedure of getting air into the lawn soil.  De-compaction is relieving the pressure between soil particles so they are less dense. For all intents and purposes they are the same and result in getting more air in to the soil. It is these air spaces that partially fill up and store the rain water for use by the grass.

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As well as relieving compaction, aeration also allows air, rain and nutrients to penetrate a hard surface, improves bacterial activity and helps reduce thatch. It also improves drainage from the surface, increases the water holding capacity and therefore stimulates rooting and root depth giving a more drought tolerant lawn. If your lawn suffers from premature drying, poor drainage or moss invasion aeration will help.