svn:externals allow a part of an external Subversion repository to be checked out within another working copy, by setting the svn:externals property on a directory in that working copy.
Externals Definitions (svn:externals SVN property) help you to construct a working copy that is made out of a number of different checkouts.
An externals definition is a mapping of a local directory to the URL — and ideally a particular revision — of a versioned directory. In Subversion, you declare externals definitions in groups using the svn:externals property.
The convenience of the svn:externals property is that once it is set on a versioned directory, everyone who checks out a working copy with that directory also gets the benefit of the externals definition. In other words, once one person has made the effort to define the nested working copy structure, no one else has to bother — Subversion will, after checking out the original working copy, automatically also check out the external working copies.
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