One of Drupal’s best features is the modular design, the core of Drupal is very small but flexible, and it only implements basic functionality. The good news is that Drupal already has hundreds of modules to implement any missing feature, you can even build your own module if you didn’t find a module to fulfill your needs. But having all these choices make finding the appropriate module a really hard job, to help you select the best Drupal modules I found a good list of module at Acquia (a commercial company founded by Drupal inventor Dries Buytaert to provide different services around Drupal).