How can I download a distribution?
A distribution is a collection of Drupal modules configured to perform a certain function.  An installation profile is a script that will configure Drupal at install time.  A distribution always has an installation profile in it, but not every installation profile will be a distribution.  An installation profile can be distributed without the modules it uses; in that instance, you can use drush make to download them, as installation profiles include a make file.

Drush dl will download distributions as well as Drupal core and contrib modules (e.g. drush dl commerce_kickstart).  Distributions are updated whenever the developer chooses to update them; of course, you can always run drush pm-update on your copy of your distribution, but there are testing concerns -- occasionally, updates will break things.  This is no different with distributions than your own Drupal install, but in the case of a distribution, presumably the dev will test and fix problems before rolling out an updated release.