As we have read, copyrighted works are "all rights reserved" by default. This means that the copyright holder (you, in our case) keeps all the rights to the work that the law provides. If someone wants to use your work in a way that isn't clearly covered by fair use, they have to track you down and ask you for permission. To save them the trouble, you can choose to apply a Creative Commons (CC) license to your work. Works that have CC licenses are "some rights reserved."
The full license contains the needed language to make it legally binding (see, for example, the full CC-BY-NC license language). Creative Commons also provides a readable, layman's summary to accompany each license. The portion that appears in your work is much more brief than either of these linked documents. Typically, you would include a notice that shows an image representing the selected license and a link to the license itself (sometimes with other information). Did you notice that I licensed this guide you have been reading? Navigate to the bottom of the page to see it. What CC license did I choose? Where does the link within my license notice take you (i.e., to the full license or to the summary)?
Once a work is licensed, it cannot be undone, even if you remove the license notice from the work or delete/destroy the work. For example, let's say you post a paper online with a CC-BY license (more on what that license means in a moment). Ahmad saves a copy of the work and posts it elsewhere online, giving you proper attribution. You then remove the CC-BY license from where you initially posted it or even remove the entire document from that website. Ahmad is (and others who find it on Ahmad's site are) free to continue to use the work as is permitted under the license you initially applied to it, CC-BY. Be sure sure you think carefully about which license you wish you use before choosing one. If you have any questions at all about which license to use for your situation, please feel free to reach out to me (see Contact Me box on this page).
So what are the CC licenses? What does each license allow people to do with your work and what are they required to do in order to use it? Read through the linked Guide to Creative Commons Licenses (this is essential reading).
When you are ready to choose a CC license for your work, use this License Chooser tool (filling out what you can of the optional section is recommended). In addition to guiding your choice, it will give you the appropriate icon, wording, and even coding (for websites) that you can use within your work to let others know how the work is licensed.
No! Applying a license doesn't mean you lose copyright of your work.* The Creative Commons licenses are a supplement to copyright and allow for specific uses with specific requirements or restrictions.
* Except for the CC0 license, which waives all of your rights and immediately puts your work into the public domain.
No! You are free to choose whether or not to license your work, and which license to use if you choose to license it. Your grade in your course cannot and will not be negatively impacted regardless of what choice you make. If you are unsure why your instructor is asking you to consider licensing your work, be sure to ask them.