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There is no difference between branches and tags in Subversion. The only difference is in what the user then does with the directory. Branches are typically created, edited, and then merged back into the trunk. Alternatively, tags are created as a snapshot of the project at a point in time and then never changed.
A tag is just a “snapshot” of a project in time. In Subversion, this idea already seems to be everywhere. Each repository revision is exactly that—a snapshot of the filesystem after each commit. However, people often want to give more human-friendly names to tags, like release-1.0 .
A trunk in SVN is main development area, where major development happens. A branch in SVN is sub development area where parallel development on different functionalities happens. After completion of a functionality, a branch is usually merged back into trunk.
The trunk is the main line of development in a SVN repository.
A branch is a side-line of development created to make larger, experimental or disrupting work without annoying users of the trunk version. Also, branches can be used to create development lines for multiple versions of the same product, like having a place to backport bugfixes into a stable release.
Finally, tags are markers to highlight notable revisions in the history of the repository, usually things like "this was released as 1.0".
See the HTML version of "Version Control with Subversion", especially Chapter 4: Branching and Merging or buy it in paper (e.g. from amazon) for an in-depth discussion of the technical details.
As others (e.g. Peter Neubauer below) the underlying implementation as /tags /branches and /trunk directories is only conventional and not in any way enforced by the tools. Violating these conventions leads to confusion all around, as this breaks habits and expectations of others accessing the repository. Special care must be taken to avoid committing new changes into tags, which should be frozen.
I use TortoiseSVN but no Visual Studio integration. I keep the "Check for modifications" dialog open on the second monitor the whole time, so I can track which files I have touched. But see the "Best SVN Tools" question, for more recommendations.
The "trunk", "branches", and "tags" directories are conventions in Subversion. Subversion does not require you to have these directories nor assign special meaning to them. However, this convention is very common and, unless you have a really good reason, you should follow the convention. The book links that other readers have given describe the convention and how to use it.
The answer by David Schmitt sums things up very well, but I think it is important to note that, to SVN, the terms 'branch', 'tag', and 'trunk' don't mean anything. These terms are purely semantic and only affect the way we, as users of the system, treat those directories. One could easily name them 'main', 'test', and 'releases.'; As long as everyone using the system understands how to use each section properly, it really doesn't matter what they're called.
A great place to start learning about Subversion is http://svnbook.red-bean.com/.
As far as Visual Studio tools are concerned, I like AnkhSVN, but I haven't tried the VisualSVN plugin yet.
VisualSVN does rely on TortoiseSVN, but TortoiseSVN is also a nice complement to Ankh IMHO.
To use Subversion in Visual Studio 2008, install TortoiseSVN and AnkhSVN.
TortoiseSVN is a really easy to use Revision control / version control / source control software for Windows. Since it's not an integration for a specific IDE you can use it with whatever development tools you like. TortoiseSVN is free to use. You don't need to get a loan or pay a full years salary to use it.
AnkhSVN is a Subversion SourceControl Provider for Visual Studio. The software allows you to perform the most common version control operations directly from inside the Microsoft Visual Studio IDE. With AnkhSVN you no longer need to leave your IDE to perform tasks like viewing the status of your source code, updating your Subversion working copy and committing changes. You can even browse your repository and you can plug-in your favorite diff tool.
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