NLTK Trees by Richard Marsden.
A number of NLTK functions work with Tree objects. For example, part of speech tagging and chunking classifiers, naturally return trees. Sentence manipulation functions also work with trees. Although Natural Language Processing with Python (Bird et al) includes a couple of pages about NLTK’s Tree module, coverage is generally sparse. The online documentation actually contains some good coverage although it is not always in the most logical location (e.g. the unit tests contain some very good documentation). This article is intended as a quick introduction, and the more informative documentation pages are listed under Further Reading.
A handy introduction to NLTK trees.