The tags on this page define different ways of saying "ignore this". These tags either tell the browser to ignore sections of code, or to treat the code as plain text, not as HTML.
Unfortunately, not only is HTML not completely standard, the methods of ignoring HTML is not standard. Of the five tags on this page, we only recommend using one:
<!-- ••• --> (and even that has its problems). The other tags are outdated, not widely supported, or just turned out to be bad ideas.