Configure how users can input content

Drupal "input filters" are a little known but powerful feature. They can give a site administrator a lot of control over the input of their site. Some contributed modules add various filters here to control content display and link behavior.
"Filters" process text - by adding special code, supporting formatting shortcuts, or even removing unwanted content.
"Input formats" are collections of individual filters, put together to let you decide what does and what doesn't happen to your text.
Input formats are usually set per role - so different types of user have more or less control over the layout of their posts.

In the Administration area, select Site Configuration > Input Formats.
(?q=admin/settings/filters)

There you can choose what happens to text or HTML that users enter. It's recommended that most users are limited to using "Filtered HTML", which is simple HTML with all the potentially problem-causing tags taken out.

Filters are managed by the filter.module
How filters and input formats work