Stemming is the reducing of words to their root. For example, removing the 'ing' from stemming to get 'stem'. However, even this example shows it is not simple. The root of 'stemming' is 'stem' not 'stemm'!
Stemming is used in many free text information retrieval systems to improve the matching algorithms.
The full text search of Human-Computer Interaction uses the most rudimentary stemming - it removes 's' from the ends of words.
There are a small number of 'stop words' which are not stemmed to prevent it from stemming words such as 'fuss', but that is all.
If you are intersted in more complex forms of stemming, see the (Internet Archive) pages for the Lancaster (Paice/Husk) stemming algorithm and more recent JavaScript implementation of it on GitHub.
