The author is the copyright holder. As the author/creator of a work, you are the copyright holder, unless and until you transfer the copyright to someone else in a signed agreement.
Tranferring copyright doesn't have to be all or nothing. Copyright is a bundle of several rights. The law allows you to transfer copyright while still keeping rights for yourself and others. The terms of your publication agreement determine what rights you give to your publisher, and what rights you retain.
Assigning your rights matters. Normally, the copyright holder possesses the exclusive rights of reproduction, distribution, public performance, public display, and modification of the original work. An author who has transferred copyright without retaining these rights must ask permission, unless the use is one of the statutory exemptions in copyright law.
The copyright holder controls the work. Decisions concerning use of the work, such as distribution, access, pricing, updates, and any use restrictions belong to the copyright holder. Authors who have transferred their copyright without retaining any rights may not be able to place the work on course web sites, copy it for students or colleagues, deposit the work in a public online archive, or reuse portions in a subsequent work.
Image credit: Wikimedia Commons / Public Domain
The law also gives the copyright owner the ability to give these rights away to someone else (like a publisher), or to share them with others by granting a license.
*Although these rights are exclusive to the copyright owner, they are not without limit, as they are specifically limited by "fair use" and several other specific limitations set forth in the Copyright Act.