What do “branch”, “tag” and “trunk” mean in SVN repositories?

I know I come after the battle, but I still would like to present my view of those concepts (under a community owned answer so I am not here for the karma ;)

• Trunk:

The main development area. This is where your next major release of the code lives, and generally has all the newest features.
The trunk directory contains the most current, approved, and merged body of work. Contrary to what many have confessed, my trunk is only for clean, neat, approved work, and not a development area, but rather a release area.
At some given point in time when the trunk seems all ready to release, then it is tagged and released.

• Branches:

Every time you release a major version, it gets a branch created. This allows you to do bug fixes and make a new release without having to release the newest - possibly unfinished or untested - features.
The branches directory contains experiments and ongoing work. Work under a branch stays there until is approved to be merged into the trunk. For me, this is the area where all the work is done.
For example: I can have an iteration-5 branch for a fifth round of development on the product, maybe a prototype-9 branch for a ninth round of experimenting, and so on.

• Tags:

Every time you release a version (final release, release candidates (RC), and betas) you make a tag for it. This gives you a point-in-time copy of the code as it was at that state, allowing you to go back and reproduce any bugs if necessary in a past version, or re-release a past version exactly as it was. Branches and tags in SVN are lightweight - on the server, it does not make a full copy of the files, just a marker saying "these files were copied at this revision" that only takes up a few bytes. With this in mind, you should never be concerned about creating a tag for any released code. As I said earlier, tags are often omitted and instead, a changelog or other document clarifies the revision number when a release is made.
The tags directory contains snapshots of approved branches and trunk releases. Whenever a branch is approved to merge into the trunk, or a release is made of the trunk, a snapshot of the approved branch or trunk release is made under tags.

I suppose with tags I can jump back and forth through time to points interest quite easily.