Filter
The filter module allows administrators to configure text input formats for use on your site. An input format defines the HTML tags, codes, and other input allowed in both content and comments, and is a key feature in guarding against potentially damaging input from malicious users. Two input formats included by default are Filtered HTML (which allows only an administrator-approved subset of HTML tags) and Full HTML (which allows the full set of HTML tags). Additional input formats may be created by an administrator.
Each input format uses filters to manipulate text, and most input formats apply several different filters to text in a specific order. Each filter is designed for a specific purpose, and generally either adds, removes or transforms elements within user-entered text before it is displayed. A filter does not change the actual content of a post, but instead, modifies it temporarily before it is displayed. A filter may remove unapproved HTML tags, for instance, while another automatically adds HTML to make links referenced in text clickable.
Users can choose between the available input formats when creating or editing content. Administrators can configure which input formats are available to which user roles, as well as choose a default input format.
For more information, see the online handbook entry for Filter module.