There is commercial software and then there is shareware. With commercial software, you have to pay for the product before you use it. With shareware, you can use the product for a trial period and then decide if you want to keep it. If you want to keep the software after the trial period is up, you're supposed to (and should) register the product and pay the shareware fee. As an extra incentive to pay for the software, many shareware programs disable certain features in the non-registered version and some will keep bugging you to register the program after the trial period has expired.
Shareware programs are usually less expensive than commercial software programs, but they are usually less expensive to develop as well. This is why shareware programs are typically not as robust as commercial software programs. However, there are numerous shareware programs out there, such as system utilities, that can be very useful. The most common way to get shareware these days is off the Internet. Check out C|net's Shareware.com for a huge selection of these great little programs.