A text editor is a program to create and edit of text files. Although it is not impossible to edit files without using one, text editors make it easy to edit configuration files. Gentoo @system contains a virtual to make sure at least one editor is installed.
More text editor options can be found online in the app-editors category or by running:
Vim and Vi
If Vim is installed the vi and vim commands become synonymous due to the following link:
ls -al /usr/bin/vi
lrwxrwxrwx 1 root root 3 Nov 25 19:59 /usr/bin/vi -> vim
The synonymous use also holds for setting editor defaults.
sudo does not preserve the user's environment variables and therefore may have a different default editor.
Due to the sensitive nature of /etc/sudoers it may only edited via the visudo command which in turn is limited to a predefined selection of editors. Type man visudo for more information.
Setting system default
The system wide default text editor can be defined in the /etc/env.d/99editor file. To change the Gentoo system default from nano to Vim add the path to Vim's executable:
The system default editor can also be set with the eselect utility. Replace
<NUMBER> in the following command with a number corresponding to the text editor of choice:
eselect editor list
Available targets for the EDITOR variable:  /bin/nano  /bin/ed  /usr/bin/emacs  /usr/bin/ex  /usr/bin/vi [ ] (free form)
eselect editor set <NUMBER>
Actually eselect modifies the file above. It seems that the old method of editing rc.conf doesn't work any more.