The Digital Millennium Copyright Act (DMCA) of 1998 updated US Copyright law in several important ways. In addition to complying with international treaties, the DMCA expanded copyright to include digital creations as well as to limit the liability of online service providers.
The full text of the law can be found on the US Copyright Office's website as a PDF.
For website creators and maintainers, the most important implications of the DMCA are:
1. All content published on the internet are covered by the same copyright guidelines as traditionally printed or distributed material.
2. Media such as videos or music may not be distributed (e.g. streamed or made available for download) without permission
3. Online service providers (including website creators) are not liable for infringement caused by other parties--like a comment posted on a blog--if they respond promptly when notified of the infringement.
US Copyright law covers use of creative works barring an agreement between the creator and the user. Creators are free to hand over all copyright to another person or entity. They may also license works to be used with certain limitations.
The Creative Commons (CC) Licenses are a set of standard licenses that creators may release their work under to allow for no-cost reuse of their content. The Creative Commons licenses provide clear guidelines on how a licensed work might be used. There are four restrictions that may or may not be placed on the reuse of a given work.
1. Attribution - The CC Attribution license requires that the content creator be credited with the original creation.
2. ShareAlike - ShareAlike requires that any derived works based on the content must be licensed under the same terms of the original. This is sometimes called "copyleft" as well.
3. NoDerivs - NoDerivs means No Derivatives may be made. The work in question must be redistributed unchanged and in while, with no changes made to it.
4. NonCommercial - The work may not be used for commercial purposes
Licenses can combine these restrictions as they like, so a work may be released requiring attribution and that it only be used for non commercial purposes, but allowing derivatives to be created and shared under other licenses, for instance.
For educational and classroom uses, all variations of the Creative Commons licenses can theoretically be used in a website or blog, though we still must abide by the Attribution and NoDerivs, and ShareAlike stipulations when applicable.
The Creative Commons Website has a wealth of information for both creators and users of CC licensed works.
Below is a list of resources to help you find resources (mostly video and images) that you can safely use in a website or blog.