HTML email is an email that is formatted like a web page using HTML, CSS, graphics, color, table columns and links

Plain text email is an email that only includes text without any coding involved

• Better visually engages users
• Easy to break up content into more digestible pieces
• Can track opens and clicks
• Better conversion rate to sales than a plain text email

• Takes more time to create then a plain text email
• Image blocking
• More chance to be placed in the spam folder if there is sloppy code
• Format is not consistent across all email clients

• An HTML email should also contain a plain text version
• Use inline-styling instead of linking am external stylesheet
• Keep the maximum width between 600-800 pixels
• Design for simplicity with grid-based layers and tables
• Don't use JavaScript or Flash in HTML emails, if animation is desired use a gif
• Use absolute links for images, so you need to host them on a reliable server
• Use basic cross-platform fonts like Arial, Verdana, Georgia and Times New Roman
• Need to consider multiple email providers and responsive design when trying to make your email consistent throughout all email providers and web browsers
• Send HTML emails to yourself to test the layout across multiple email providers and web browsers

Resources:
MailChimp HTML Email Basics
MailChimp HTML to Text Email Converter