<header> is an HTML5 element that defines a section of a webpage that usually contains the introductory or navigational content, and it is typically placed at the top of the page. It can contain other elements such as headings, logos, navigation menus or search fields.

is a heading element in HTML that represents the main heading or title of the page. It is usually the largest and the most important heading on the page. It can be used only once on a page and is typically placed after the <header> element.

In summary, <header> is a section of a webpage, while

is a heading tag used to define the main heading of the page.