Forms are useful tools for your website. Here are a few ways to use them.
For forms that need to use the flexibility of drupal fields, rules, etc: https://drupal.org/project/entityform. These are more complex to setup, but very powerful.
For multiple choice, long answer, etc quizzes: https://drupal.org/project/quiz. This is limited in its expandability and only really helpful for quizzes.
For things that don't need to be embedded in a site, and that you'd like to tie to google docs, you can use: http://www.google.com/google-d-s/createforms.html
If you're doing things far more complex than that you may want to look at external services like: https://www.surveymonkey.com
If none of these fit the bill then you can build something custom in drupal, you just need to know the form api:
Form Generation: https://drupal.org/node/1702548
Form submission: https://api.drupal.org/api/drupal/includes!form.inc/group/form_api/7
Altering forms: https://drupal.org/node/1702628
Form API Reference: https://api.drupal.org/api/drupal/developer%21topics%21forms_api_reference.html/7
You can then tie your webform or custom forms to outside services if you'd like using the following: