how nutch plugins work?

Question

I am new to nutch, but i know nutch uses Lucene for indexing,which only understands text format.

Nutch have many plug-ins that can is used for crawling the particular format that plug-in meant for. my doubt is how actually the nutch plug-in works?.

I seen the Team wiki page for nutch

i want some information like how actually nutch works with lucene.

Thanks you.