make your text bold
<b>YOUR TEXT</b>

 www.ily637.com rocks!

make your text italic 
<i>YOUR TEXT</i>

 www.ily637.com still rocks!

strike through your text
<s>YOUR TEXT</s>

ily637.com effin' rocks!

underline your text
<u>YOUR TEXT</u>

ily.637.com rocks a little more.

 _sub-script your text
_{YOUR TEXT}

 ily637.com _{rocks your face} off.

<br>
causes a line break

  ily637.com<br>rocks.
would look like this:
ily637.com
rocks.

 make your text bold and italic
<b><i>YOUR TEXT</b></i>

 ily637.com rocks for these tutorials.

 make your text italic and underlined
<i><u>YOUR TEXT</b></i>

 ILY637.COM is tired from rocking so much!