A directory tree that contains mostly symbolic links to files in a master directory tree of files. Link farms save space when one is maintaining several nearly identical copies of the same source tree - for example, when the only difference is architecture-dependent object files. They also mean that changes to the master tree are instantly visible in the link farm. Good text editors provide the option to replace a link with a new version of the target file when saving thus allowing the farm to have its own versions of just those files that differ from the master tree.
Link farms may also be used to get around restrictions on the number of "-I" (include-file directory) arguments on older C preprocessors. However, they can also get completely out of hand, becoming the file system equivalent of spaghetti code.