We've discussed projects and series, which are the major ways in which we keep track of the progress of a free software project in Launchpad.
In general, most of Launchpad's tools allow you to group information at the level of the series. For example, different series in the same project can have different translations, and you can plan to fix bugs in a given series or implement blueprints in a series.
Launchpad allows you to have strict control over the bugs and blueprints that are planned for fixing or implementing in a given series. If you want to, you can require that a project driver approve each bug or blueprint that is linked to a particular series. In this way you can ensure that you are only making commitments that the project can keep. This is quite a heavyweight management approach, but it's useful for large projects like Ubuntu. Smaller projects often just turn that control off.
By contrast, milestones are a very lightweight way to organise a group of bugs or blueprints
A milestone is a point in time, or a test release, for which you need to keep track of a few bugs or blueprints. In Launchpad, you can easily create a milestone, and then link bugs or blueprints to that milestone as a way of saying "we think these items are worth keeping track of as we get closer to that date".
Here's an example, the 0.15 milestone of Bazaar:
At the time of writing, it looked something like this:
Milestones are useful as a quick way to focus attention on a subset of bugs or blueprints which are important over the next week or month, before a specific point in time, or before a test release is made. They are not as rigorous in terms of control: anybody can throw a bug onto the list, or a blueprint. However, they are very useful for smaller projects, or projects that don't have the resources to plan major releases.
Thank you for taking the time to review all of these highlights in Launchpad! There is a lot more, beneath the hood, and it does take time to come to grips with an infrastructure this large.
Launchpad is constantly evolving, so please join us on #launchpad on irc.freenode.net or on the launchpad-users mailing list. We are always working to improve Launchpad for Ubuntu, and hope that it will be useful to other projects as well.