I've just changed the licence my blogs are published under. Everything that's published in most of the world is automatically covered by copyright and by default the author has all rights to their words (or images) and also by default nobody else has the right to republish the work. Some people choose to allow others to republish their works with or without modifications and there are a number of licences that give these permissions, they are all based in and depend on copyright law.
If you are going to use a free licence you need to carefully choose one that allows your work to be used in the way that you are happy with while blocking uses you consider inappropriate. "Carefully" both to give effect to your wishes and because all the free licences I'm aware of are perpetual and once you've licenced someone to use your work they can continue using it under that licence for the life of the copyright.
The free licences I'm most familiar with are the Creative Commons but there are alternatives. Since the first of April 2005, 4 months after its start, this blog has been under a creative commons licence, but it was an old licence that wasn't ported for New Zealand law and I've been planning to update to a current licence for a while. My other blogs were under a mix of different licences and in some cases even non-CC restrictions. Yesterday I decided to do something about it and discovered this licence chooser on the Creative Commons site.
When I dialed up the current version of the licence I'd been using, I was surprised that it said "This is not a Free Culture License". "What" I asked myself "is a 'Free Culture License'?" so I followed the link to the Definition of Free Culture Works site and read what it said about my preferred Non Commercial restriction. Among other things it said "there is no generally agreed definition of where the border line is between Commercial and Non Commercial uses with very many cases falling in the undefined area in between" - a problem I'd encountered when wanting to reuse CC-*-NC works on my advertising supported sites: Does the advertising make the use commercial? There is a definition of Free Culture Works on their site and it talks about freedoms that are very similar to the freedoms in the Free software definition. As a concept it made a lot of sense to me and I decided I was more comfortable publishing under a Free Culture licence.
After a bit of thought I decided the Creative Commons Attribution-ShareAlike 3.0 New Zealand License (CC-BY-SA) was right for me. Attribution means you have to acknowledge my authorship and I ask for a link to the original page as that attribution. Share Alike means that it's copyleft, like the GPL, if you republish you must do so under the same (or very similar) licence.
At the top of this post I said "once you've licenced someone to use your work they can continue using it under that licence for the life of the copyright." This obviously applies to the earlier postings in this blog as well. Anyone who is already using content from before the change from here under the original licence (and obeying its provisions) is free to continue using it under that more-restrictive licence. I was happy enough with it when I chose it and until I found the Free Culture site only wanted to update it.
Note to spelling pedants: I'm writing in New Zealand English and "Licence" is the correct spelling here. Where I use the word "License" above it is because I'm quoting something in US English.