TEXT: is the first half of the email source. this is displayed if the email client is set to TEXT ONLY, or cannot display HTML. this method is not common, but is still provided.
HTML: is the second part of the email source. this is displayed when the email client can display html in emails. this is the modern way if displaying email and is most common.
if you open an email to view its true source (not the web page source), you'll see that there are two versions of the email. first is TEXT, followed by HTML
the email client usually displays the HTML version if it can.
You may remember way back prior to 2005, in early forum software, there would be a user option to manually select "email type: text only. html only." In modern email clients, the client is usually who chooses this.
Furthering the case of using only plain text emails, either the recipient would have turned HTML off for incoming emails, or someone at their company or email host has. There are some companies that have strict no-HTML policies for security and privacy reasons.
