Creative Commons Attribution
Creative Commons Attribution license (or CC-BY) is an open license, which allows reuse of a creative work (e.g. writing, images, audio or video) for any purpose provided attribution is given to the original author. The license (in a "human readable form") can be found at Creative Commons Attribution 3.0 Unported - different versions exist for different languages and legal regimes.
Being an open license, reuse may occur for
- commercial or non-commercial uses
- whether the original author approves or not
The downside of this is that someone could reuse the content, modify it, and then not share - for example using it in copyrighted works, or in a more restrictive open license (CC-BY-SA, GFDL, or FAL) or the "not quite open" CC-BY-NC-SA, any of which would prevent the original user from using the adaptations in their own work. In this sense, the CC-BY license is the most generous of open licenses, but offers the least safeguards for those who care about such things. However, attribution remains a requirement in all cases - this will not be attractive to someone who does not want to share credit at all - so in theory, a license requiring attribution offers as much protection as copyright does against use without attribution.
In practice, those reusing the work may misunderstand the license - is this an issue, and is it any worse than than the improper use of copyrighted material which happens anyway?
Marking your work
Marking your work is important to ensure it is indexed as open content by search engines. (E.g. in Google, click advanced search and select usage rights).
Creative Commons provide a way of doing this - see Marking your work as Creative Commons.