Drupal comes with its basic core modules. We can however add contributed (external 3rd party) modules and plug it in to our Drupal website, to get more features and extensibility. Note that these are not core modules. They are mainitained by 'outside' people and may not be up todate, may require further security updates or may not meet later versions of Drupal core. In other words, principle of caveat emptor applies. They are generally however, safe provided you signup for the security announcements. There are many many contributed modules.

We can also add many other external 3rd party themes im similar manner. So we can actually have more than 6 themes.

Finally, we can also write our own module and or theme and plug in if we know how.

For beginners, do not worry. There are some very 'common' modules that are very stable and most people use.

Some important concepts when adding external themes / modules.

1. Never amend the original core modules and themes.

2. Always use the /sites folder and park the external modules and themes in this path. The /sites folder by default is like this:-
/sites/all/README.txt
/sites/default/setting.php

3. This /sites folder is where we create our website(s). Say we 3 have websites, we create 3 sub-folders under /sites:-
/sites/my_domain.com
/sites/mom_domain.com
/sites/dad_domain.com

4. Copy /sites/settings.php to all the 3 sub-folders, create 3 databases/user and manually configure each settings.php to bind the database and url.
/sites/my_domain.com/settings.php [configured with say database db_my]
/sites/mom_domain.com/settings.php [configured with say database db_mom]
/sites/dad_domain.com/settings.php [configured with say database db_dad]

What is all means above is that one Drupal installation can run mutiple websites.

5. Not only that. And each website can have different themes/modules and or common themes/modules.
Say if we want to use the feedback (external) module and make it available for all 3 sites, we put this modules in the /sites/all folder like this:-
(a) we create a sub-folder named 'modules' in /sites/all:-
/sites/all/modules
(b) upload and extract the feedback module here, it becomes like this:-
/sites/all/modules/feedback/[files_are_here]

6. Or say only 1 website i.e www.mom_domain.com is providing online shopping and needs the external ecommerce module, then we create sub-folder 'modules' in /sites/mom_domain.com (only for this site) and put like this:-
/sites/mom_domain.com/modules/ecommerce/[files_are_here]

7. If we want to add an external theme say the 'Slash Theme', and if we want to make this 'slash' theme available for all sites:-
(a) we create a sub-folder named 'themes' in /sites/all:-
/sites/all/themes
(b) upload and extract the slash theme tarball here, it becomes like this:-
/sites/all/themes/slash/[files_are_here]

The above is very powerful concept. We are able to mix and match different features/modules for different websites, all running on one instance of Drupal!