So, stemming, what is stemming? Generally speaking, stemming is finding the basic form of a word. For example, in the sentence "he walks" the verb is inflicted by adding a "s" to it. In this case the stem is "walk" which, in English, also happens to be the infinitive of the verb.
We will first present a few examples of stemming in natural language, and since Dutch is my native language I will concentrate on Dutch examples.