Despite what some people believe, information published on the web is not automatically in the public domain. Yes it is available for everyone to be able to read but that isn't what being in the public domain means. Being in the public domain means that you can do whatever you like with the material and very little of what is published on the web falls into that category.
Almost everything on the web is in fact fully covered by copyright just the same as if it were published any other way and copying anything from the web without the permission of the copyright holder is stealing. The key to being able to copy any material from the web is to make sure that you have the permission of the copyright holder to use the material for your specific purpose. This requires that you correctly identify the copyright holder, let them know what you want to copy and why and then get their written permission to copy it for that purpose.
Just because an article or image on the web does not contain a copyright notice does not mean that the material is in the public domain. Only material that explicitly states that it is in the public domain should be considered to be available for you to use however you like without needing to first get permission. You do need to be sure though that the material actually is in the public domain and that the copy you are seeing that states that it is in the public domain isn't itself stolen from the true copyright holder.
There may be a notice published on the web site granting permission to use their material in specific ways. Where they grant that permission you can of course take that as your written permission to use it in the way specified provided that you are sure that they actually own the copyright on the material in order to grant that permission in the first place. You need to be particularly careful when copying material from a site that can be updated by almost anyone as the person who uploaded the content you want to copy may have stolen it in which case the copy on that site is illegal to start with and they can't grant permission to copy something to which they themselves do not have the right to display in the first place.
Where you are granted permission for a specific use the permission that you have is limited to that specific use and you will need to get permission again if you wish to use it for something else.
Some people may be thinking about "Fair Use" rules that permit copying without the permission of the owner but many people attempting to apply that rule do not properly understand it. The rule limits the uses for which you can copy the material and unless you know that you are in a situation where fair use applies (such as your working for a news service or educational institution) then the chances are that you have no right whatever to copy any of the material under the "Fair Use" rules. Even where those rule do apply the amount that you can copy is limited to a very small fraction of the whole (eg. not more than one chapter from a book of 10 or more chapters or not more than 10% of the whole thing where the article or image has fewer than 10 chapters). Even there you may be required to prove that your use of the material actually does fall under the fair use provisions. I actually teach at a local college and every so often they request a complete copy of all of the material that I hand out in my classes so that they can check that any material for which I do not own the copyright or have the permission of the copyright holder actually is short enough and relevant enough to satisfy the fair use provision (they ask for the copy despite the fact that all of the material that I use was written by myself as they need to actually verify that so as to protect themselves from copyright breach claims).
The presence or absence of a copyright notice on web pages does not affect whether they are covered by copyright at all. The only affect including such a notice has is to advise some of the people that don't understand about copyright that the owner is interested in protecting their copyright. That a notice doesn't appear doe not mean that they are less interested in chasing thieves, it just means that there is a greater likelihood of thieves.
The safest thing is to always create your own content and to never use any content belonging to anyone else. I have heard of people employing someone to write articles for them and subsequently finding out that the article was stolen. That you purchased it from the thief doesn't protect you at all when the real owner takes you to court for its theft.
This article written by Stephen Chapman, Felgall Pty Ltd.