Lets start with copyright infringement.
It is easily explained as copying someone else's work without gaining permission first. This includes publishing other peoples work to your web site, newspaper, magazine or even just downloading it to your computer can be seen as copyright infringement. The last is known as a grey area.
I have just finished reading a book on how to service my car. The author of the book explains in detail the different ways a person can go about the job. He also details tips and tricks he has learnt over the years that speed up the servicing.
Now after putting into practice what the author wrote I could go and write my own book on how to service my car, right?
Why? Well the author can only copyright the content of his book (words, pictures... etc) he cannot copyright the practice or idea there in. This can be viewed in many different ways so if your unsure it is always best to at least email the author before taking any action with there material.
The short and sweet way of looking at this is your content no matter what creative medium it falls under is automatically copyrighted to you for the duration of your life. It can only be broken if you decide to give your content away for free (you must state that it is copyright free material) or you die and nobody registers for the copyright to your material.
This leads us to Public Domain material.....
Before 1978 copyright only lasted 27 seven years. These days it lasts the duration of the creators life. Therefor from the minute you create your work it is automatically copyrighted to you. It is still important to state this wherever you decide to publish your material.
Public Domain material has no copyright and can be used in anyway you see fit. Also unless you change it in someway it is still Public Domain. So if you find a book that is in the Public Domain and you start selling it on EBay, there is nothing stopping the people who buy it from selling it themselves. That's why it is important to change the material. Add something to it so it stands out or modernize the language so people can understand the material better. Any changes copyright the "new" book to you.
Anything published before 1928 falls under Public Domain. That is of course unless someone got there first and changed the material in some way. You have to watch out for that. It could get you into trouble.
That's the basics. For a more detailed insight into copyright go to GOV.
Also depending on your country things maybe different so make sure you know where you stand.
Copyrighting your work is a good thing. You are the sole owner of what you create and so you should be. There is a problem here though. Once your material is published to the web, book, magazine or even TV your leaving your material open to abuse.
This could be viewed as unfortunate but truthfully if it wasn't for copying other peoples HTML code I would have never learned HTML. It was the same for Java Script and Graphic Design. Even writing and game creation were learnt from copying others first.
It is also well known that internet marketers keep something called a Swipe File on there computer. This is where they paste items of interest they find on the web. It is a digital scrapbook basically.
People learn by first copying and then creating. It is the natural order of life to copy others. So if you want to be strict about your copyright that's fine. Just remember how you learnt what you know before you draw a line in the sand.