Though Everybody has replied here in their way and all are right , sorry for late reply here,

I am telling the meaning and the purpose of BB code.
Basically the BBCode Is the Short Form Of "Bulletin Board Code" and this code is specially used in all forums only. Its just like <HTML> but html is more powerful than BB code and its the lightweight markup language.
The available tags are usually indicated by square brackets surrounding a keyword, and they are parsed by the message board system before being translated into a markup language that web browsers understand—usually HTML or XHTML.

:arrow: Its Main Purpose Is As Follows:
BBCode was devised to provide a safer, easier and more limited way of allowing users to format their messages. Previously, many message boards allowed the users to include HTML formatting, a side effect of which was that malformed HTML could disrupt the page's layout, or HTML could be used to run JavaScript leading to XSS attacks. Some implementations of BBCode have suffered problems related to the way they translate the BBCode into HTML, which could negate the security that was intended to be given by BBCode. An alternative is a properly written HTML filter.
Although the basic tags of BBCode are similar in most internet forum software, there is currently no standards document for BBCode and there are consequently many incompatible variants. Some variants are case-sensitive while others are not. Some variants have tags which highlight the formatting of HTML, ASP, PHP, SQL and other markup languages and programming languages.

Some Sample Of BBcode ( I think Its not required here as you always using BBcode here in ur post.)



Code:
&#91;b&#93;&#91;color=yellow&#93;BBCode&#91;/color&#93;&#91;/b&#93;

&#91;b&#93;bolded text&#91;/b&#93;
&#91;i&#93;italicized text&#91;/i&#93;
&#91;u&#93;underlined text&#91;/u&#93;
&#91;s&#93;strikethrough text&#91;/s&#93;
&#91;url&#93;http&#58;//example.org&#91;/url&#93;
&#91;url=http&#58;//example.com&#93;Example&#91;/url&#93;

I think writing about HTML is not required here as you all are very familiar with this markup language and you also very good at html so i am not describing about html here, if you need it then plz tell me i will again write about it.
Thanks.