There're various kinds of different redirect operations you can do using .htaccess

301 (Permanent Redirect)

Redirect an entire site to a different URL on a permanent basis.
For example, to redirect all your site to newdomain.com

Redirect 301 / http://newdomain.com/

302 (Temporary Redirect)

Redirect an entire site to a different URL, but not on a permament basis (it's useful for maintenance and for SEO purposes)
For example, to redirect all your site to newdomain.com

Redirect 302 / http://newdomain.com/

Redirect an old file to a new file path

This is useful if you reorganize your site, changing the URL, paths and names of files, but want to keep all of your linking in, redirecting the visitor directly to the new path.
You can have also hundreds of these redirects command in your .htaccess, so it amy be useful also if you changed your site from old, useless file names to new, SEO friendly file names; being so, you'll need to insert a command row for each file to be redirected.

Redirect /olddirectory/oldfile.php http://mydomain.com/newdirectory/newfile.php