Ferret improves Vim's multi-file search in four ways:

Ferret provides an :Ack command for searching across multiple files using The Silver Searcher, Ack, or Grep. Support for passing options through to the underlying search command exists, along with the ability to use full regular expression syntax without doing special escaping.

Shortcut mappings are provided to start an :Ack search (<leader>a) or to search for the word currently under the cursor (<leader>s).

Results are normally displayed in the quickfix window, but Ferret also provides a :Lack command that behaves like :Ack but uses the location-list instead, and a <leader>l mapping as a shortcut to :Lack.

Finally, Ferret offers integration with dispatch.vim, which enables asynchronous searching despite the fact that Vim itself is single-threaded.

The companion to :Ack is :Acks (mnemonic: "Ack substitute", accessible via shortcut <leader>r), which allows you to run a multi-file replace across all the files placed in the quickfix window by a previous invocation of :Ack.

The quickfix listing itself is enhanced with settings to improve its usability, and natural mappings that allow quick removal of items from the list (for example, you can reduce clutter in the listing by removing lines that you don't intend to make changes to).

Additionally, Vim's :cn, :cp, :cnf and :cpf commands are tweaked to make it easier to immediately identify matches by centering them within the viewport.

Finally, Ferret provides a :Qargs command that puts the files currently in the quickfix listing into the :args list, where they can be operated on in bulk via the :argdo command. This is what's used under the covers by :Acks to do its work.

For more information, see the documentation.