From this link,
https://wincent.com/wiki/@executable_path,@load_path_and@rpath

Note: this article is actually about the @executable_path, @load_path and @rpath install paths used by the linker on Mac OS X; wiki titles can’t include underscores, however, because they are ambiguous with spaces.
Absolute paths

Useful for frameworks installed in shared locations. Example:

Install path: /Library/Frameworks/Foo.framework/Versions/A/Foo

@executable_path

Useful for frameworks embedded inside applications, because it allows you to specify the location of the framework relative to the application’s executable:

Install path: @executable_path/../Frameworks/Foo.framework/Versions/A/Foo
Application location: /Applications/Foo.app
Executable path: /Applications/Foo.app/Contents/MacOS
Framework location: /Applications/Foo.app/Contents/Frameworks/Foo.framework
Linker puts all this together to figure out that the framework binary can be found at: /Applications/Foo.app/Contents/MacOS/../Frameworks/Foo.framework/Versions/A/Foo

@loader_path

Available from Mac OS X 10.4 Tiger onwards; useful for frameworks embedded inside plug-ins, because it allows you to specify the location of the framework relative to the plug-in’s code (remember, plug-ins may not actually know where they are going to be installed, relative to the application, so knowing @executable_path doesn’t help us in this case):

Install path: @loader_path/../Frameworks/Foo.framework/Versions/A/Foo
Application location: /Applications/Foo.app
Plug-in location: /Library/Application Support/Foo/Plug-Ins/Bar.bundle
Executable path: /Applications/Foo.app/Contents/MacOS
Loader path: /Library/Application Support/Foo/Plug-Ins/Bar.bundle/Contents/MacOS
Framework location: /Library/Application Support/Foo/Plug-Ins/Bar.bundle/Contents/Frameworks/Foo.framework
Linker puts all this together to figure out that the framework binary can be found at: /Library/Application Support/Foo/Plug-Ins/Bar.bundle/Contents/MacOS/../Frameworks/Foo.framework/Versions/A/Foo

Note that if the “loader” is an application rather than a plug-in, the @loader_path ends up being equivalent to @executable_path.

@rpath

New in Mac OS X 10.5 Leopard is @rpath. Key points:

@rpath instructs the dynamic linker to search a list of paths in order to locate the framework
critically, this list is embedded in the loading application
this means that a single framework with @rpath/Foo.framework/Versions/A/Foo can be made to work in a number of different ways; that is, you are effectively no longer limited by the choice of specifying your "install path" using either @executable_path or @loader_path
the down side: you now have to pass additional linker flags when building the host application (eg. -rpath @executable_path/../Frameworks or /Library/Frameworks; note that specifying both will cause the dynamic linker to try looking in both locations)

Sources

Nice overview: http://www.mikeash.com/pyblog/friday-qa-2009-11-06-linking-and-install-names.html
Docs for using PFiddlesoft Frameworks (I’ve never used these, but the manual itself makes some nice general points about using frameworks): http://pfiddlesoft.com/frameworks/downloads/Assistive_Application_Programming_Guide.pdf