When you create something, it belongs to you. Giving creators exclusive rights to original work creates an incentive for people to continue to create and share new work.
Ideas are not under copyright. Once something becomes "fixed tangible expression," they are automatically considered the intellecutal property of the owner.
Copyright protects the creator, but eventually, copyright expires, and all work enters the public domain. This means it belongs to the public. In the United States, creative works move to the public domain 70 years after the creators death. Public Domain works belong to the public. More on Public Domain.
Creative Commons licenses allows content creators to give permission to reuse more recent work for free. There are several types of Creative Common licenses. Some allow the work to be distributed, and some allow the work to be remixed, adapted, and built upon. Attribution is almost always required.
If you want to use something under copyright that does not fall under fair use, you may be able to get permission from the creator. More on Permission and Licensing.
Copyright and Attribution
Fair use is the legal use of material without the copyright owner's permission. This is allowed in specific situations, especially educational and editorial use.
More about fair use