Fields on comments are defined at the content-type level, on the <em>Comment fields</em> tab of the content type edit page (which you can reach from the <a href="@types">Content types page</a>). When you add a field for comments, each comment on a content item of that type will have that field added to it. For example, you could add a website field to the comments on forum posts, to allow forum commenters to add a link to their website.