WordPress will only prompt you for your FTP connection information
while trying to install plugins or a wordpress update
if it cannot write to /wp-content directly.
Otherwise, if your web server has write access to the necessary files,
it will take care of the updates and installation automatically.
This method does not require you to have FTP/SFTP or SSH access,
but it does require your to have specific file permissions set up on your webserver.
It will try various methods in order, and fall back on FTP if Direct and SSH methods are unavailable.
http://core.trac.wordpress.org/browser/tags/3.1/wp-admin/includes/file.php#L866
WordPress will try to write a temporary file to your /wp-content directory.
If this succeeds, it compares the ownership of the file with it’s own uid,
and if there is a match it will allow you to use the ‘direct’ method of
installing plugins, themes, or updates.
Now, if for some reason you do not want to rely on the automatic check for which filesystem method to use,
you can define a constant, 'FS_METHOD' in your wp-config.php file that is
either 'direct' 'ssh', 'ftpext' or 'ftpsockets'
and it will use method.
Keep in mind that if you set this to ‘direct’ but your web user
(the username under which your webs server runs)
does not have proper write permissions, you will receive an error.
If you do not want to (or you cannot) change permissions on wp-content
so your web server has write permissions, then add this to your wp-config.php file:
define('FS_METHOD', 'direct');
To maintain a higher level of security, Rolet servers set the permissions on the
/wp-content directory to read only for apache.
Although this can be changed via SSH, FTP and through your control panel,
we highly recommend that you enter the FTP credentials each time you install or update a plugin.
The server and credentials can be remembered by your browser to make it quicker
to install plugins and only requires an extra click.
If your website is hosted on Rolet’s cPanel servers, the FTP credentials
to enter will be the same username and password combination
as your cPanel account or any other FTP account with access to the /wp-content directory.