301 Redirect and DNS change

What is the different between 301 Redirect and DNS change, becuase i want to redirect my url to nonwww to www.

if you redirect your page via .htaccess or header or...
your browser does this:
check DNS->goto URL(redirected)->goto URL(get homepage)

if you change the DNS:
check DNS->goto URL(get homepage)

that's about the difference.

So you want to force a "www." before your domain?