Copyright
"Copyright is a form of intellectual property which gives the creator of an original work exclusive rights for a certain time period in relation to that work, including its publication, distribution and adaptation [...] Copyright applies to any expressible form of an idea or information that is substantive and discrete." (From Wikipedia)
In general one can say that there is no Copyright problem when all materials (pictures, graphics, music, photographs, texts, etc) were specifically created by yourself or specially for you. In most other cases you can use materials published under certain (free) Creative Commons Licenses or other free or paying, commercial licenses. In the latter case you may need explicit permission from the owner or creator of the materials before these materials can be used. Breaches of Copyright laws can lead to high fines and penalties, even when those materials are used in an educational context, and therefore Copyright needs to be treated with great care.
More information about Copyright can be found on Wikipedia's page on Copyright.
“Creative Commons is a nonprofit corporation dedicated to making it easier for people to share and build upon the work of others, consistent with the rules of copyright.” They “provide free licenses and other legal tools to mark creative work with the freedom the creator wants it to carry, so others can share, remix, use commercially, or any combination thereof”. See also specific Creative Commons divisions for research and education: ccLearn and Science Commons.
