Nemo is a file manager for Linux that is a fork of Nautilus. In recent versions of GNOME or Ubuntu, Nautilus has been simplified and lots of features have been removed. For example, I like to use Compact View and this has been removed in the Nautilus in Ubuntu 14.04. If you need a file manager that is similar to the old Nautilus, but having a good set of features, then pick Nemo.
If you are using an older version of Ubuntu, add the PPA:
$ sudo add-apt-repository ppa:webupd8team/nemo $ sudo apt-get update
Installing Nemo is the same in all versions of Ubuntu:
$ sudo apt-get install nemo
If you would like to make Nemo your default file manager, please see this post.
Tried with: Nemo 2.0.8 and Ubuntu 12.04