CSS codes where place after the head tag of your html file. Internal Stylesheet will be used to one particular page only. Internal Stylesheet code format is something like this.
Now External Stylesheet all your css codes will be saved into an extension file ".css" that file contains css codes only no "html,body,head" tags only css. You use External Stylesheet if you want all your pages will be look the same. You link your css file something like this.
<link rel="stylesheet" type="text/css" href="mystyle.css" />
And I rather to say if you want your css codes to be use by multiple pages without writing them all over again then use External StyleSheet.