The terms copyright and public domain are mainly connected because they are two sides of the same coin. An intellectual work is generally either considered to be under copyright, or a part of the public domain. If something is under copyright, that means the creator still has many exclusive rights to it. On the other hand, if the work is in the public domain, it means that anybody can make use of the work with relative freedom. Copyright and public domain laws may vary a lot internationally, and there are some people who feel copyright laws are too restrictive or unnecessary.
The main thing that separates copyright and public domain is often financial. If a creator puts something into the public domain, he or she is generally offering to give it away, perhaps as a gesture of good will, or an attempt to contribute to the available public knowledge pool. If someone decides to assert his copyright privileges, in many cases, he might be looking to profit from the work in the future. Newer copyright laws generally assume that new creations are under copyright unless the creator says otherwise.
Most works will generally move between copyright and the public domain over time. Many countries have laws that prevent things from staying under copyright indefinitely. For example, in the United States, most works from the early part of the 20th century are now considered part of the public domain, and for this reason, they might be freely available over the Internet and legal to use for nearly any purpose.
There are "fair use" laws that provide certain legal ways to make use of things that are under copyright without getting the creators' permission. For example, using intellectual property for the purpose of reviews, news, and parody is usually protected. There are also often special protections given for the use of works in an educational context. These laws usually allow enough wiggle room for people to argue about the exact lines where fair use crosses into the realm of copyright infringement, and lawsuits might occasionally occur on this basis.
Many countries have agreements with each other when it comes to copyright and public domain issues. For example, there are treaties that allow a person from one country to assert his copyright privileges in another country if he thinks his work is being used in an unlawful way. Not all countries recognize these agreements, and there is generally no way for an author to protect his work under all circumstances.
Some people feel that copyright and public domain laws need to be totally abolished, and others think they need to be seriously overhauled. These people have many different reasons for their stances. Some feel that copyright laws are impractical in the modern Internet era, while others feel that copyright laws may stifle creativity or funnel artists towards commercial instead of intellectual pursuits, which they believe might be detrimental to society as a whole. On the other end of the spectrum, many people hold the opinion that copyright laws are necessary to give artists an incentive to create.