HTML stands for Hypertext Markup Language, meaning that you mark up text with special tags to indicate function. These are done through <>. Common tags you may see are <table> <tbody> <tr> <td> <p> <div> <span> <a href=”URL”> <div> <img /> <br />. Everything in HTML is a defined word, such that <p> will always be paragraph, and <paragraph> does not have the same function or meaning.
Most tags require a corresponding closing tag, so for example you’d open a table with <table> and then you’d close it with </table>. However, other tags are self-closing, like <br /> which means it includes the “/” inside the opening tag to close itself.
HTML is rendered, or displayed, in a program like a browser (Internet Explorer, Chrome, or Firefox, etc.) or an email client (Like Gmail, Hotmail, Outlook, Yahoo, Apple Mail, gmail app). Browsers are typically up to date and standards compliant, but email clients are not. Thus, there are some HTML strategies that work for emails, that don’t work well in browsers, and vice-versa.
You may want to use www.litmus.com (or other similar services) to test emails in different email clients and see how it will display. There is often a discrepancy between how an email will appear in the browser and how it will appear once rendered in a specific email client.