Bazaar is a source control system we use in house as an alternative to CVS and SVN. Source control is what helps multiple developers work on a project without continually overwriting each others' work. Before Bazaar, we were just asking each other which files we were editing and trying to remember when to stay out. This actually worked quite well, but some fixes and features would occasionally turn up missing.
We host our Bazaar repository at http://bazaar.ubercart.org. If you browse there, you can see a log of committed changes and a small form to pull one up by ID (if you're really interested). There are also two links near the top that will let you browse the repository or download the latest zipped version of the code (updated every hour).
To find Bazaar downloads and documentation, head to the official website:
We're still newbies when it comes to Bazaar, but we invite you to join us in figuring it out! Our latest code is kept in a Bazaar repository with anonymous read access. To get it, you need to download and configure Bazaar and then use one of the following commands:
bzr checkout bzr://bazaar.ubercart.org/drupal5/ubercart
bzr checkout bzr://bazaar.ubercart.org/drupal6/ubercart
This will download the latest code for Drupal 5 or 6 to an ubercart folder in your current working directory. Sorry I'm not more Bazaar savvy than that... I just write code and trust it gets to the right place eventually. 
You can track our progress by viewing a log of code commit messages at http://bazaar.ubercart.org. If you want to update your code, you can use the following command from within the ubercart directory:
bzr update
So, for the initial checkout, let's say you're putting Ubercart in your sites/all/modules directory, you'd want to go to that directory and use the checkout command. Then to update later on, make sure you're actually in sites/all/modules/ubercart before doing it.
We're using it on both our WinXP production machine and Linux server, so take your pick from the Bazaar downloads section.
There are some tutorials and things in the Bazaar docs, and I found section two of this fairly helpful: Bazaar for CVS Users. The help pages in the Bazaar program are also very useful:
bzr help
(1-14-08: Please note that if you were checking Ubercart out in alpha and can no longer checkout/update the code, you should review this thread on switching to the new repository location.)