Usability testing is the measurement of the usability of a product or service. The definition from usability.gov succinctly describes the process.
In a usability test, representative users try to do typical tasks with the product, while observers, including the development staff, watch, listen, and take notes. The product can be a Web site, Web application, or any other product. It does not have to be a finished product. You should be testing prototypes from early paper-based stages through fully functional later stages.
Source: "Learn About Usability Testing." Usability.gov
Note that the process of usability testing differs from usability engineering, which is concerned the research and design of usable products, rather than testing a product's usability.
You will need to find usability test participants. To source them, you have different options ranging from selecting from your pool of current users, to recruiting them yourself or through an outside service.
There are commercial reports and seminars available for those interesting in quickly getting started with recruiting. Jakob Nielsen's Nielsen Norman Group provides an extensive report of 233 tips for helpful recruiting, and Jared Spool's User Interface Engineering offers an on-demand seminar on recruiting.
You can recruit your own users by pooling contacts using your customer databases and screening them using any combination of emailing and phone, web survey and interview.
If you want to automate the process partially, http://ethnio.com/Ethnio provides a service to recruit, screen and manage user research participants form your own website.
WebUsability.com provides a list of resources that may be helpful in sourcing recruits.
For professional recruiting services, the firms below will work with you to source participants based on the criteria you provide (e.g. demographic, psychographic and technographic information, etc.)
Usability Testing Software
This is an example of the types of software that are commonly used for usabilty testing. For more tools, please see the Usability Testing Tools page.
- Silverback, by Clearleft. (Local testing)
Hosted Remote, Unmoderated Usability Testing Services
- User Testing (Uses self-identified paid test participants.)
- Open Hallway