The moment you write something original in idea or expression on the back of a napkin, in your journal, or any other sheet of paper (or any other textile or surface) - or input it into your computer, you own the material. Unless you sell your copyright to someone else (i.e. a Work Made for Hire Agreement).
Outside of that nebulous area called "Fair Use" no one else can publish your material without your permission. You created it; you own it. When publishers offer you a book contract (and "book" is very inadequate term to convey what they want), they are purchasing your permission to own exclusive sales, distribution, territorial, and publishing rights to your material. Publishing rights means they have all control over the printing of your work, whether on paper with ink, whether in audible voice, whether in dramatic presentation, whether in workbook form, whether in electronic medium - or in any other medium that exists now or will in the future exist in all the universe. And so forth. (Get the idea?)
But YOU will still own the copyright. It is your intellectual property. You just can't do anything with that property. Unless you reserve certain rights, you no longer are allowed to do anything with your material that is no allowed by your publisher. If you want to donate three chapters to your church for a ministry booklet, that's fine - if and only if it's fine with the publisher.
One of the classic historic battles between writers and publishers was over copyright ownership. Even into the 90s (and yes, this Century), many boilerplate contracts indicated that the publisher was acquiring ownership of the copyright and that the book would be copyrighted in the publisher's name. That battle is mostly over, with most publishers agreeing to register a book with the U.S. Copyright Office (or the country of origin) in the author's name.
But I thought I didn't have to do anything to copyright my work? Why would a publisher go to the trouble?
There are some smaller publishers who actually don't go to the trouble and in most cases, it won't be a big deal. It won't change the legal standing to the work. But registering the material is an action that conveys a publisher is going to protect the copyright, which is a huge issue.
Protecting copyright is the source of much acrimony and confusion in the world. As an example, I lived in a city where a local high school copied a university's trademarked logo (a trademark is different than a copyright, but you get the idea) for their football helmets. The university, after learning of the violation after several years of use, issued a cease and desist letter. The moral outrage and outcry by supporters of the high school team was loud and sometimes vicious - and wrong. If the university had not protected their trademark in this instance, they would lose the ability to control something essential to their identity and possibly lose millions of dollars in licensing fees in the future.
Does that mean you can't let others use your material? Of course not, but I wouldn't recommend it without requiring proper attribution, including the (c) designation with your name. In a church bulletin? Yes. As a chapter in someone else's book? Definitely. If you don't protect it that way, why would a publisher offer you money for it at a later date? Be generous all you want, but be consistent in protecting your ownership.
Bible publishers have done a good job of granting generous permission for authors and organizations to use the material from their translation, in many cases at no charge, but always with the requirement of proper attribution and copyright notification. Outside of the King James and a few other public domain translations, there will be specific guidelines set forth in the front matter of your Bible or on the publisher's website. Check it out as a good case study.
There are a host of subplots surrounding the topic of copyright. I've already mentioned Fair Use, which deserves its own blog and is still too slippery to nail down. There's subrights issues, international and U.S. differences on the term of a copyright, tricks for extending copyright beyond its expiration date, review rights, Work Made for Hire issues, serial rights, and other nuances. This blog is in no way exhaustive, but is at least highlighting one simple application for you as an aspiring published author: protect your property.
How? You don't have to put (c) Your Name on all your work. But why not do so anyway as an initial precaution. Make sure you establish when you created your work in case someone claims that you borrowed or stole from them. Let others enjoy and use your work before you are able to turn it into a payday, but only with proper attribution and notification - and any other conditions you would want to stipulate. And when you have a publisher ready to buy your work, make sure you understand exactly what you are selling. If you are a new author, the publisher is going to want to buy all rights from you to make sure he or she can "exploit" those rights in any way necessary to make your deal profitable for both parties. (Exploit sounds awful but it's not a bad word in this context!)
The small things can save you big problems later. If you think disagreements over physical property gets brutal, wait until you see a fight over something that is a product of the mind!