1These tags are discouraged, for stylistic reasons. And <pre> should be eschewed in favor of <code> (see next).
2<code> and <c>, used for displaying code/data, are not true HTML tags, but are interpreted by the PerlMonks engine. They inhibit the normal interpretation of enclosed HTML special characters like <, >, &, [, and ]. Any newlines in the enclosed code will be rendered such that long lines wrap. Like HTML, <code> and <c> tags are case ignored. Unlike HTML, whitespace is not allowed inside (e.g., < code > will not be recognized), and they must have a matching closing tag (</code> or </c>) for them to work. It is not possible to enclose </code> within a <code> block, nor </c> in a <c> block; they would be taken as block closers instead.
3readmore, used for abbreviating very long posts within the context of other posts, is not a true HTML tag, but is interpreted by the PerlMonks engine. A readmore block is rendered as a <div class="readmore"> block when viewing the node directly, and as a "Read more..." placeholder link otherwise. The link is to a page where you are viewing the node directly, thus displaying the full content. Unlike HTML, whitespace is not allowed inside (e.g., < readmore > will not be recognized). If a matching closing tag (</readmore>) is not given, the readmore block is taken to extend to the end of the body of your post.
5The <wbr> HTML element is used to indicate a suggested word-break point for wrapping long strings with no whitespace. See documentation at the W3C.