InputFilter has a number of levels - if you use NoMarkup it simply htmlencodes the content, which is why you're seeing the angle brackets and other characters encoded. This means that the html is effectively converted to plain text- when it's displayed in a browser it will not be interpreted as tags. If you use NoScripting it searchs for potential dangerous strings to remove, this allows developers the option to allow their users to use HTML but protect against common xss/html injection attacks. Where possible you should always use NoMarkup, unless you absolutely need your users to have the capability to use html.

Cathal