What is the difference between HTML and plain text?

I have the feeling I should know this answer, but have no idea.

Just started using a new emailer, Thunderbird, having used OE for years.

Now, when I compose mail, I am asked whether I want to send in:
1. Plain text only.
2. Plain text and HTML.
3. HTML only.

OE never asked. It just sent the mail.

I have no idea what the differences are. Sometimes I want something in bold or italics, for emphasis, or I want a different font and size.

Which of the three choices should I select?

Take for example this page.If you are using Internet Explorer then go via View/Source and this will open a text file which contains the HTML code for this page.The level of detail shown in HTML far surpasses that available in text and can include images etc along with different writing styles.Outlook Expresses default is to respond in the same form of email it recieves if it recieves HTML it usually responds in HTML.
I hope this helps :D

104456:
Your explanation helps and confirms my earlier impression. The problem is that lots of sources say it is just a waste of time (because the recipient may not receive it as such) or HTML opens up the possibility of someone hijacking my computer, or it takes up too much bandwidth, etc.

I just want to be able to put something in bold ,or italics , or in a different font size when I reply.

Am I asking too much?

aob--I know nothing about Thunderbird, but I am afraid you are going to have to use HTML or Rich Text to keep the formatting. (In OE you made your choice of setting under Format on the toolbar and/or from Tools|Options|Send tab|Mail Sending Format. That is why OE did not ask you. I would assume TB also has some way of making a setting default and not asking each time.)
But since you are asking about sending messages, why are you worried about security?
What the recipient sees depends on how he/she has set up the email settings. For example, in OE, you (or the recipient) can set Tools|Options|Read tab to "Read all Messages in Plain Text" and then, indeed, you will not see any formatting in the messages.

welshjim:

Have now selected HTML as the default for Thunderbird and hope that is the end of it.

Thanks much.

aob--Yes, indeed. Good luck. Let us know if this does not do what you want. But remember, you cannot control what the recipient sees if they have set their email program to read only Plain Text. :)