Nemo is a fork of Nautilus that keeps some of the features that were removed in Nautilus for the 3.6 release of GNOME. This post gives a good overview of some of the other reasons why people might want to change to Nemo over Nautilus -- http://cinnamon.linuxmint.com/?p=198
Installing the Nemo File Browser
Luckily, in Fedora 18, nemo is available in the Fedora repositories, so installing it is as simple as searching for nemo in Add / Remove Packages, or running:
yum install nemo
from the command line.
Once installed, you will be able to run nemo as a secondary file browser. Just search for nemo in the overview. It should show up as "Files" with a Folder icon, rather than the Filing Cabinet icon of Nautilus.
Using Nemo as Default File browser
Setting up GNOME to replace nautilus with nemo as the default file browser is a little bit more involved. First you need to start up the gnome menu editor, alacarte (yum install alacarte if you haven't previously installed it).
Once you have alacarte running, in the right hand pane, choose the Accessories category. The right hand pane should list two items with the Label "Files".

Disable the nemo entry (the Files entry with the folder icon) by deselecting the checkbox beside the folder icon. Next, edit the nautilus entry (the one with the Filing Cabinet icon), by selecting it, and choosing properties.
In the Dialog that pops up, change the command from "nautilus %U" to "nemo %U"

This should result in GNOME bringing up nemo rather than nautilus in most circumstances.
hope this helps,
ryanlerch
Since the question has pretty much changed, would you be able to tweak the title and description to read something closer to "Changing default File Browser from Nautilus to Nemo" or similar?