Comments and other text you can post on iRi use standard HTML, with some helpful additions. I'll start with a quick primer on HTML, then tell you what you can use on iRi.
iRi uses HTML despite the fact it's not the easiest language, because it's the only standard language you may be able to carry your knowledge of elsewhere; see Please Stop with the HTML Replacements for more on that.
In the simplest case, you can just type text in paragraphs and it will work. The Preview will show you how it will look, and unless you encounter a problem with special characters like & or <, or you want to add some formatting, that should be fine.
HTML is based around the idea of tags, which can label text. For instance, I made the word "tags" italic by adding <i> to the front of "tags", and </i> to the end. The first tag "opened" the italics, and adding the slash to the front of the tag "closed" the tag.
Tags should be closed in the reverse order they were opened. <b><i>text</i></b> is legal, <b><i>text</b></i> is not. iRi will make your HTML legal, but that may not do what you intend.
For each of these tags, surround the text in the tag as shown in the example.
Blockquote (best way to quote something)</blockquote>
- <tt>Teletype Text (fixed-width)</tt>
pre (retains formatting, best for code)</pre>
Links are done with the a tag, using the href attribute:
<a href="http://your.link/here.html">Link Text</a>
You can also use abbr and acronym with the title attribute.
This is not a complete list of permitted tags. iRi is pretty permissive. (No img tags for you, though.)
Entities (<, >, and &)
There is something in HTML called an entity, which is how HTML allows you to use special characters in HTML, including the characters special to HTML. If you use a <, >, or & character in a way that is not valid HTML, iRi will try to guess whether you meant to use the entity instead, but it is impossible to always guess correctly. You may need to explicitly replace < with <, > with >, or & with &.
You can also use all defined HTML entities such as ™ (™), but you can also use Unicode in your posts, and that is probably preferable for Unicode characters. (In fact such characters get converted to Unicode entity references, like ™ for ™.