What is a html email?
Html email simply looks like a web page with all the colors and graphics. You can see an example of html format email here.
What is an email client?
Email clients are the scripts/software used to retrieve, read and manage your email. Email clients can be 'Web based email clients' and 'PC based email clients'.
Web based clients: You can login into an online web based panel where the script retrieves the email from your mail server and presents infront of you. Examples: Squirrelmail Roundcube Zimbra Citadel
PC based clients: You can install the software on your computer that retreives email from your mail server. No matter which one you use, Windows or MAC, there are many free email clients for you to choose. Examples: Eudora Microsoft Outlook Thunderbird Pegasus
So all these email clients display Html emails?
Some of them need us to enable display of html in emails. Ex: Squirrelmail.
There is a link 'Options > Display preferences' in your Squirrelmail panel. You can enable or disable the html display.
Some clients display html by default. They can strip away the javascripts and images to keep your computer safe. Ex: Hotmail. Hotmail removes images and asks us whether to display those images or not. Hotmail strips away javascript code too. If html option is not enabled in your subscriber email clients, then the html email looks like this: See the raw html code ditecly displaying in the email client instead of rendered and displayed ...

As an email marketer what is your way out?
You are not sure whether your subscriber has enabled html in his email client. So sending html email will not serve your purpose of sending email if your subscribers have not enabled html.
An average person who doesn't know anything about html emails just deletes your email from inbox thinking that ununderstandable code might contain viruses. If you as an email marketer don't know how to deal with this problem, then you will end up in more unsubscriptions, spam complaints or moving your email into junk folder.
* You can offer the subscribers the option to select text or html during subscription process with 'Text' option selected by default. This way a person don't know anything about email formats can safely be receiving your text format emails.
* Send emails as MIME messages where both text and html email can be sent in same email and if subscriber email client allows html display, html email will be shown. Otherwise text version is shown.