According to the United States Department of Labor, "ergonomics is the science of fitting workplace conditions and job demands to the capabilities of the working population." When done well, ergonomics can improve worker productivity, avoid injury, and improve worker morale.
An important note: the term "ergonomic" is not federally regulated and can be used, without penalty, by anyone. This unfortunately means that not all products that use this term are truly ergonomic.
Ergonomic products should fit your body, your work environment, and the purpose for which they will be used. Movement, as in changing positions, is a key principle that comes into play when creating a work environment. Little things, such as taking a break and going for a short walk, stretching, standing part of the day, switching mousing hands, or changing the position of your chair can really help increase comfort, and reduce the risk of injury.
An ergonomic office design should bring all your tools into positions that allow your body to work with the least amount of stress. In other words, as relaxed as possible. Twisting, contorting, and reaching should be minimized.
If you are having significant pain issues, it is in your best interests to see a professional. A true ergonomist can examine your work environment, how you work, and your current tools and make personalized recommendations based on their education, experience, and the facts that are specific to you. While an ergonomic assessment is not inexpensive, it may well prove to be an extremely smart decision that could prove priceless over time if the pain issues are eased or resolved, and disability prevented. The cost of not being able to work could be far higher.
All prices are in USD.