The header tag introduces a part of a content like with an image or a menu for the whole page.... You may have multiple headers like an header for each article.
Usual attributes
Attribute name
Attribute value
class
Set a value for CSS rules. The CSS rule will be applied to all tags with the same class name
id
Set a unique value, it is useful for JavaScript or for CSS rule. For CSS it will be applied only for this tag
style
Set a CSS value. It has always priority for general CSS rules. Sample : "background-color:red;color:white"