The FindPackages function.
The functions in this package are for searching and maintaining Mathematica packages. FindPackages will give a list of the packages available, in their context form (e.g., "Utilities`Package`") or in a full path name form (e.g., "/usr/local/Wolfram/Mathematica/AddOns/StandardPackages/Utilities/Package.m".) You can specify a list of directories to search. You can also specify a string pattern, or a list of patterns, to be matched. Note that a pattern should not try to match the ".m"; the function handles this.
This loads the package.
This gives all packages whose names end in "ca" for each directory in $Path. Directories containing no packages fitting the pattern give empty lists.
Option for FindPackages.
Here the same search is performed, but full path names are returned.
The Annotation function.
Mathematica packages contain various annotative comments tagged with keywords. The Annotation function allows you to discover what keywords are present in a package (or list of packages), then retrieve the actual annotation comments containing the keywords. This function recognizes a package name in either context form or as an explicit file name.
This finds the annotative keywords in the Statistics`NonlinearFit` package.
The string "Mathematica Version" is a useful keyword; it generally indicates the minimum release of Mathematica required for a particular package.