Without Software your system would be very limited in what it and you could do. Many PCs will have at least half a dozen different types of Software application ranging from Office and email applications to messenger programs and graphic packages some which are listed on this page.
Operating Systems - These are essential to handle the various systems and inputs into a PC. Microsoft dominates with the Windows range, but alternatives namely many Linux Distributions are growing in popularity.
Office Suites - Again Microsoft's Office Program dominates the market, alternatives such as Open Office and Lotus Notes are available. Many large companies and individuals use alternative products as the license fees are far lower and they offer the same functionality as the more expensive Office product.
Email Programs - Several popular programs to choose from. Microsoft products are either Outlook aimed at the more business user and Outlook Express for the home/office user. These are bundled either with Windows or the Office suite. Free alternatives include Mozillas Thunderbird and Others including Eudora, both are excellent programs and I would highly recommend them.
OEM - Often only available through resellers or when buying an item of hardware. This software may be an operating system or other application. It will typically come without any retail packaging or support and is often at a reduced price to the full retail version.
Open Source - Software which is released under the Open Source guidelines allows access to the source code for anyone who puts in the time to work on making a program or script better without requiring payment.
Many people have written small programs and they offer them to the online community for free, although the author retains the copyright and it can not be altered or sold.
Shareware this is similar to Freeware except often the software is only free for an evaluation period. To continue to use it you may need to pay a small fee which may include updates and support. Many programs will stop working if you have exceeded the trial period.