How are opening and closing tags different?

An opening tag is used to indicate the starting point of an action by a tag. The closing tag shows when to stop this action. The front slash is added to a tag, before the tagname, to indicate that it is a closing tag. What is between the tags is effectively selected for this action. To start bolding for example, we use <b> and to stop we use </b> as follows:

This text is <b> bold </b> text.

The Word bold in the above sentence would be bolded, as that is what is enclosed by the opening and closing tags. The text outside of that is as normal.