Yes, I suppose finding only information like "This routine applies the Porter Stemming Algorithm to its parameters, returning the stemmed words" in the stemmer docs could be a little confusing to someone who is just trying to find out what a stemmer does.

What a stemmer does is fairly straightforward. It extracts the stem or root of words, no matter what form they are in. For example, a text like "He gets up, takes his pills, all the while leaving the water running.", a stemmer would return the stems of all the words in the sentence: "he get up take his pill all the while leave water run".

Stemmers often make mistakes with ambiguous forms like 'running' in "his running improved his circulation". Here 'running' is a noun, but a stemmer may treat it as a verb and return 'run'.

Interesting links:

• Cultured Perl: Writing Perl programs that speak English
• Parsing Natural Language with Lingua::LinkParser