Copyright is what it says it is, the right to make copies of a written work. Copyright covers a finished form of expression -- book, movie, etc. It does not cover an idea. There is no protection for ideas.
Copyright is automatic. As soon as you write down an original work, you own the copyright on that work. The copyright declaration is simply a piece of information to the reader telling them who owns the copyright. It does not create the copyright and the copyright is just as valid with or without it. It basically says, this is who you ask if you want to make copies.
Each country had their own copyright laws, but most countries are signatories to the Berne Convention which establishes basic principles of copyright law and binds each signatory country to respect the copyrights of the citizens of other signatory countries. In other words, your copyright is automatically protected in all signatory countries as soon as you write your work down.
Some countries will let you register a copyright to prove you were the originator of the work. Some people think you should do this to protect yourself against people wanting to steal your work. But this fear is unfounded. Publishers and agents are looking for writers, not individual works. If you can write a story worth publishing, they want to sign you as a writer, not steal your stuff. And Tony Soprano can make more money running protection rackets than stealing unpublished manuscripts.
If you post your stuff on the Web, occasionally some numbskull will come along and copy it to their site. Some think they are doing you a favor by giving you more exposure. Some are just trying to generate traffic. They mostly don't know copyright law from a poached egg. Suing them would be more trouble that it is worth in most cases. A letter threatening to report them to the hosting provider will usually suffice. Best advice: don't post fiction on the Web unless you are simply intending to give it away. All you are doing is using up some of the rights you were hoping to sell to publishers.
The only real criminal use of copyright is when some disgruntled unsuccessful writer spots some superficial resemblance between their work and the work of some big hit author like J. K. Rowling and tries to sue them for copyright infringement. But by the time you are a big enough name for these people to be want to sue you, you will have the money to fight them.
The real function of copyright, therefore, is not to protect your from manuscript thieves, it is simply the thing you have to sell to publishers. Publishers buy "rights" from authors, which basically means the right to make and sell copies of their work in certain jurisdictions for a certain period of time. An author may sell the copyright outright to a publisher for a fixed sum, but the more usual arrangement is that the author retains copyright and licenses certain specific rights to make and sell copies. This is why you need an agent: to negotiate those rights for you.
Copyright is not forever. At a certain point, the copyright in a work ends and it reverts to the public domain, meaning that anyone can then make and sell copies of it. This is why there are a hundred different copies of classic novels on Amazon for 99 cents a copy -- someone downloaded the text from Project Gutenberg, made an ebook, and put it on Amazon. However you will be long dead before your copyright runs out. (Rules may be different in different countries, but basically copyright belongs to you from the moment you write it till you die, at which point it passes to your heirs and remains in force for many years afterwards.)
Your exclusive right to make or to license the making of copies of your work are limited by the "fair use" doctrine. This says that people may quote a certain amount of your work in their work for the sake of criticism and various other purposes. By the same token, if you want to quote pieces of other people's work in yours, you can only do so up to the limits of fair use or you will have to ask for their permission, for which they may or may not charge you money.
Copyright law also includes the right to make derivative works. If someone wants to turn your novel into a musical, they need your permission. If you want to turn their musical into a novel, you need their permission.
If someone pays you to write something for them, this is considered "work for hire" and they own the copyright, not you. The rules for when something is considered work for hire are quite specific and you should look up what they are for your country.
Beginning writers often get mixed up between copyright and plagiarism. Plagiarism is the academic sin of presenting someone else's work as your own. It is important to remember that if you copy a section of someone's work and attribute it to the original author, you are not guilty of plagiarism, but you may still be guilty of copyright violation if your use is not covered by the fair use doctrine. Plagiarism can get you fired or kicked out of school. Copyright violation will get you sued.
As a beginning author, there are three basic things to remember about copyright.
- It is yours automatically as soon as you write your work down.
- Don't copy other people's stuff, even with attribution, except as provided for by fair use. Don't make derivative works based on their stuff either.
- Copyright is the thing you own and can sell. Get an agent to negotiate the rights you sell to your publisher.
Finally, I am not a lawyer. This is the layman's summary you asked for, not authoritative legal advice.