Plain Text is simply text just like you would type it somewhere. It has no formatting to it. No font selections, no colors, no bold, underlines, etc.
HTML (Hyper Text Markup Language) is the language of an internet browser. It contains all the formatting commands necessary for the browser to understand how to display the text on the user's computer. What you're looking at now is in HTML. Your browser has converted it to a page that you can see. All the colors, logo, icons, etc were sent over the internet in HTML and your browser converted it.
If you want to see what the raw HTML looks like, go to the top of your browser now and click VIEW, then click Source. You will see what the HTML for this page looks like.
When something is entered in HTML it can be displayed on anyone's computer using their browser. Text entered some other way would require the recipient to have similar software. If you created a MS Word document someone else would have to have Word to be able to see it. (Or they would need some other program capable of reading a Word document). Using HTML saves everything in a common format that everyone has.
If I understand you correctly, you don't have to use MS Word or any other word processor. You can just enter the text directly in your Mail program. For example, if using Outlook Express, start a new message, then go to Format and be sure the HTML option is checkmarked and not the Plain Text option. When you send the mail, it will go out as HTML. You don't see the HTML code itself, but only the 'result'. At the bottom you can click the Source button if you want, to see how Outlook Express is creating the HTML for you. Note that with HTML all your formatting is also transmitted. This includes fonts, colors, bold, underline, italic etc. If instead you were using plain text, all the formats would be completely lost.
You don't need to use MS Word as you can create your HTML code directly using Outlook Express. But if you want to use Word you can. Type your document and use whatever formatting you want to use, such as font, colors.
Save the Word document. Then save it again, but select Save As HTML. (You must save it first before saving in HTML so it will preserve the formatting)
Your newly saved document will have an .HTM extension to it indicating it's an html document. Say you saved something called MyProject.HTM into your MyDocuments folder. If you just open the file directly, you'll see that what you had is now being displayed using your browser to show it, just like any other internet page would be displayed.
To put your Word's HTML text (and coding) into your email, open Outlook Express, start a new message, enter the send to and message title information, then click in the box for the message itself to activate it. Next at the top select INSERT and click Text From File. Find your HTML document that you saved (you'll have to change the filetype selected from TXT to HTM so it will show up in your listing), and select that file to insert. It will be inserted along with all the HTML code formats for it.
After insertion, you can go back to your new mail, at the bottom change from edit to source, and see what all the HTML looks like.
This may all seem complicated, but it's really not. HTML is simply the code behind the text message. The code that gives all the format commands and tells a browser how to display it. It will do much more too, but that's probably more than you'll need to know right now.
Remember, you don't NEED to use MS Word or other word processor. Just set your e-mail to save what you type as HTML and you're done. Your e-mail is going to the Professor in HTML format. You've probably been doing that for all your regular email and didn't know it. If you want to use Word instead, then follow the steps above so that the HTML formatting from the Word document is saved as part of your HTML email.
Hope this helps.