From a developer's point of view, a free software project is in a state of continuous release. Developers usually run the latest available code at all times, because they want to spot bugs, and because they follow the project closely enough to be able to stay away from currently unstable areas of the feature space. They often update their copy of the software every day, sometimes more than once a day, and when they check in a change, they can reasonably expect that every other developer will have it within a day or two.
How, then, should the project make a formal release? Should it simply take a snapshot of the tree at a moment in time, package it up, and hand it to the world as, say, version "3.5.0"? Common sense says no. First, there may be no moment in time when the entire development tree is clean and ready for release. Newly-started features could be lying around in various states of completion. Someone might have checked in a major change to fix a bug, but the change could be controversial and under debate at the moment the snapshot is taken. If so, it wouldn't work to simply delay the snapshot until the debate ends, because another, unrelated debate could start in the meantime, and then you'd have wait for that one to end too. This process is not guaranteed to halt.
In any case, using full-tree snapshots for releases would interfere with ongoing development work, even if the tree could be put into a releasable state. Say this snapshot is going to be "3.5.0"; presumably, the next snapshot would be "3.5.1", and would contain mostly fixes for bugs found in the 3.5.0 release. But if both are snapshots from the same tree, what are the developers supposed to do in the time between the two releases? They can't be adding new features; the compatibility guidelines prevent that. But not everyone will be enthusiastic about fixing bugs in the 3.5.0 code. Some people may have new features they're trying to complete, and will become irate if they are forced to choose between sitting idle and working on things they're not interested in, just because the project's release processes demand that the development tree remain unnaturally quiescent.
The solution to these problems is to always use a release branch. A release branch is just a branch in the version control system (see branch), on which the code destined for this release can be isolated from mainline development. The concept of release branches is certainly not original to free software; many proprietary development organizations use them too. However, in closed-source environments, release branches are sometimes considered a luxury — a kind of theoretical "best practice" that can, in the heat of a major deadline, be dispensed with while everyone on the team scrambles to stabilize the main tree.
Release branches are pretty much required in open source projects, however. I have seen projects do releases without them, but it has always resulted in some developers sitting idle while others — usually a minority — work on getting the release out the door. The result is usually bad in several ways. First, overall development momentum is slowed. Second, the release is of poorer quality than it needed to be, because there were only a few people working on it, and they were hurrying to finish so everyone else could get back to work. Third, it divides the development team psychologically, by setting up a situation in which different types of work interfere with each other unnecessarily. The developers sitting idle would probably be happy to contribute some of their attention to a release branch, as long as that were a choice they could make according to their own schedules and interests. But without the branch, their choice becomes "Do I participate in the project today or not?" instead of "Do I work on the release today, or work on that new feature I've been developing in the mainline code?"
The exact mechanics of creating a release branch depend on your version control system, of course, but the general concepts are the same in most systems. A branch usually sprouts from another branch or from the trunk. Traditionally, the trunk is where mainline development goes on, unfettered by release constraints, and, say, the first release branch, the one leading to the "1.0" release, sprouts off the trunk. (The details of how to create and manage branches in your particular version control system are beyond the scope of this book, but the semantics are roughly the same everywhere.) Note that you might want to name the branch "1.0.x" (with a literal "x") instead of "1.0.0". That way you can use the same minor line — i.e., the same branch — for all the micro releases in that line.
The social and technical process of stabilizing the branch for release is covered in the section called “Stabilizing a Release”. Here we are concerned just with the high-level version control actions that relate to the release process. When the release branch is stabilized and ready, it is time to tag a snapshot from the branch (see tag or snapshot) with a name like, e.g., "1.0.0". The resultant tag represents the exact state of the project's source tree in the 1.0.0 release (this is useful when developers need to compare against an old version while tracking down a bug). The next micro release in the same line is likewise prepared on the 1.0.x branch, and when it is ready, a tag is made for 1.0.1. Lather, rinse, repeat for 1.0.2, and so on. When it's time to start thinking about a 1.1.x release, make a new branch from trunk.
Maintenance can continue in parallel along both 1.0.x and 1.1.x, and releases can be made independently from both lines (while new development work happens either directly the main trunk — in Git, the "master" branch — or in short-lived "feature branches" that get merged into the main trunk as soon as they're ready).
In fact, it is not unusual to publish near-simultaneous releases from two different lines. The older series is recommended for more conservative site administrators, who may not want to make the big jump to (say) 1.1 without careful preparation. Meanwhile, more adventurous people usually take the most recent release on the highest line, to make sure they're getting the latest features, even at the risk of greater instability.
This is not the only release branch strategy, of course. In some circumstances it may not even be the best, though it's worked out pretty well for projects I've been involved in. Use any strategy that seems to work, but remember the main points: the purpose of a release branch is to isolate release work from the fluctuations of daily development, and to give the project a physical entity — the release branch — around which to organize its release process. That process is described in detail in the next section.