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.