Input format is a standard option for all node types, which determines how the certain fields should be formatted by Drupal. The most common one being the text field representing the Body of a page or story.
By default, there are at least three options available. More options can be added by the Drupal developer and in addition, which options available to your are determined by your individual permission settings.
Input format options are assigned to users and you can only choose from those made available to you.
Default options available for input format:

• Filtered HTML: This is the default option used for accepting content from the Internet (the general public)
  • Drupal will remove any dangerous code and restricted from text entered by the user once they submit the node. This ensures that information submitted by the public will not hack your website or cause problems for your website visitors
  • This format enables users to enter a limited number of HTML tags such as: <a> <em> <strong> <cite> <code> <ul> <ol> <li> <dl> <dt> <dd>
  • Lines and paragraphs will break automatically without the need of a HTML <br> tag
  • And optionally, images can be entered into the text as a image filter tag. Which is a special tag that Drupal translates into an image
• PHP code: This option is not available to end users and limited to Drupal developers only. This allows in-line code to be inserted into the website which executes and performs database lookups, image manipulation etc. It is very powerful and dangerous, therefore access is strictly limited
• Full HTML: This is the default option used for WYSIWYG text areas. The WYSIWYG widget outputs HTML tags and therefore the Full HTML option must be enabled for all of the WYSIWYG formatting to be preserved

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