Although your Blogger posts are typed via the post editor, or sent via email using the Mail-to-Post feature, these are considered to be plain text HTML documents, just as if you were maintaining a website and producing single HTML documents for all the pages in your site. So, when it comes to discussing the structure of an HTML document and how to type HTML, there's little difference between how web developers and bloggers use this code.
HTML tags are the words you see in angle brackets, such as <html> and </html>. In fact, that was an example of an HTML tag pair. Tags are either paired or self-contained, meaning there is no ending tag. The image tag is an example of this: <img> does not have a matching </img> end tag. Instead, the <img> tag itself is closed by the forward slash at the end of the single tag: <img/>. You'll learn more about specific tags in a moment. For now, just understand that tags are usually in pairs, sometimes not, and are just words surrounded by angle brackets.