When rendering a partial, a parital has access to all instance variables generated by the controller method. So if you were rendering a method from show_comments, @my_crazy_comment would be available to the partial. But not only that, if the partial is called with
<%= render :partial => "comment_display", :object => @my_crazy_comment %>
The partial would have available to it, a variable called 'comment_display' that points to @my_crazy_comment.
So inside your partial file, "_comment_display.rhtml"
<%= comment_display.comment %>
<%= comment_display.author.name %>
The variable "comment_display" will have the comment in @my_crazy_comment. For a collection of partials, it's just as easy.
<%= render :partial => "comment_display", :collection => @all_crazy_comments %>
Where @all_crazy_comments is an array of comments.