There are, essentially, as many different types of public domain software as there are types of software itself, though the number of public domain programs has decreased in recent years. It is important to keep in mind that software that is freeware or shareware, or even open source, is not necessarily part of the public domain. In general, however, there are numerous software programs including operating systems (OS), graphics editors, and word processing programs that are in the public domain. Most public domain software programs were created with the purpose of being public domain, rather than naturally moving into the public domain through copyright expiration.
Public domain software is basically the same as any other type of software, except it is not protected by copyright ownership. A copyright is a form of intellectual property ownership granted to the creator of an original creative or artistic work, over the work that was made. Computer software is typically included in this category, usually as a form of literature or written expression, and is therefore protected under copyright laws in many countries. Public domain software is software that is not protected as the intellectual property of a person or company and can be used freely in any way desired.
Most public domain software is created for release without copyright, since expiration of a copyright usually takes several decades and most computer programs are fairly modern. These software programs can include operating systems or core codes for an OS, picture editing and illustration programs, and word processing software. Since there is no copyright ownership over that software, someone who owns a copy of the program can even sell copies of the program for commercial use. This aspect of public domain software separates such programs from other software that may be available as open source programs or freeware.
An open source program is a piece of software that others can use and modify under the guidelines of the open source license that accompanies that software. Unlike public domain software that can be used in any way desired, open source programs can have stipulations or conditions for use. Freeware refers to programs that can be downloaded and used free of charge, but may also include limitations on how such programs may be otherwise distributed or shared with others. Similarly, shareware can be shared among various users, but is still typically owned by the creator. While there are still public domain software programs available, many programmers have begun creating freeware and shareware to distribute free software and still retain copyright ownership of the program.