Can I use HTML?
That depends on whether the administrator allows you to; they have complete control over it. If you are allowed to use it, you will probably find only certain tags work. This is a safety feature to prevent people from abusing the board by using tags which may destroy the layout or cause other problems. If HTML is enabled you can disable it on a per post basis from the posting form.
What is BBCode?
BBCode is a special implementation of HTML. Whether you can use BBCode is determined by the administrator. You can also disable it on a per post basis from the posting form. BBCode itself is similar in style to HTML: tags are enclosed in square braces [ and ] rather than < and > and it offers greater control over what and how something is displayed. For more information on BBCode see the guide which can be accessed from the posting page.