The difference between HTML emails and plain text emails is, well, the fact that the former contain HTML code + text, whereas the latter contain only text.
- HTML emails allow you to use different colors and fonts, boxes and sizes, images and links. Upside: graphics. Downside: some email clients reject HTML.
- Plain text emails do not contain HTML elements. They consist of simple text. Upside: they are accepted by all email clients. Downside: no graphics.