When magic_quotes is enabled, and you post data from a form, any fields containing characters such as ' will automatically be escaped.
For example if you have an filed like <input type="text" name="text_field">
and you enter a Mc'Donals on it, when you php file receives it, it will create a variable $text_field with the content
Mc\'Donals
If magic_quotes is disabled, it will contain the data as it was posted Mc'Donals
