The commons-httpclient-3.1.zip is just a zipped file of the working directories for the project, pretty typical of Apache toolkits. It is only the big applications like
Tomcat that have special installers.
I unzipped the entire set of files, using the base directory that is in the zip file so I now have a commons-httpclient-3.1 directory in my JavaMisc directory I use for this sort of thing. Inside that we find:
commons-httpclient-3.1.jar - you need this on your classpath to compile and run programs using the toolkit.
a bunch of license .txt files and RELEASE_NOTES.txt - the notes give the history of the project as leading to the current version
the docs directory - containing:
.apidocs - the standard JavaDocs formatted API documentation
.. several other directories containing examples and files used by the:
.html files that duplicate the Apache web site information including the page for the tutorial
Bill