The Core Team maintains stock, out of the box demos of the latest version of CiviCRM for each of the three content management systems with which the software integrates. These are also used to test for specific bugs found by the community (learn about reporting a bug). Note that other people are likely exploring on the demo sites at the same time as you, so they may be configured strangely, missing functionality or appear in different languages. To prevent things getting too 'strange' we reset all demo data once every day. We advise against entering personal data on these sites.
To keep everyone safe, these demos have some limitations: you can't send emails from them; you can't set permissions for Drupal users, and you can't fully explore online payment options.
If you are interested looking at the very latest, unreleased functionality that we are currently working on, you can have a look at our development sandboxes. The functionality you see on these sandboxes normally finds its way into the next version of CiviCRM. They are particularly useful if you an interested in getting a sneak peak at new features or want to help test or document an upcoming release.