There are three licenses that enable free use of software. It helps to know the difference before choosing software.
Open source
Open source is a development method where software's source code is available for open public collaboration at no cost. Some examples include Open Office, Linux, Apache, the web server, and Gimp, a Photoshop clone.
With open source you are free to make any change to it. If you choose to distribute the software, you are required to make the new source code freely available. The software code is often copyrighted or protected under Creative Commons.
Open source, freeware and shareware are often used interchangeably. The terms are not interchangeable because the requirements are very different.
Freeware
Free software is just that. It's free.
Anyone can download it and use it. However, the users usually don't have access to the computer code, and making changes to the software is prohibited by U.S. copyright law.
Shareware
Trying shareware is similar to purchasing a car. You wouldn't buy a car unless your drove it first. Software authors allow free downloads of software with a trial period, usually 30 days. At the end of the trial period, the software may stop working, or certain features may be disabled.
To continue using it, buy it. Shareware is less costly than commercial software. Some of the most important PC software used today began as shareware.
~Emery Jeffreys