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
It is not a copyleft license - that is, it does not have a share alike clause to restrict reuse to compatible licenses.
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?[expansion needed]