Usability is when something just "feels right". You don't need to pick up a manual to learn how to use a product that has great usability. You can go to a usable website and navigate around without getting confused. Similarly, you can jump into a Ferrari and feel the wow factor. Design is a form of usability and Ferrari are the kings of "car usability".
When something is usable you don't have to think how to use it. This means there is very little to no learning curve, which grows your confidence in the company that created the product. This confidence may ultimately lead to you purchasing other products from that company because you've come to expect a high level of usability from past dealings.
Compare this to being a web designer. You build a visually appealing and usable website. If someone visits that site and sees your "Powered by" link at the bottom, don't you think there's a huge chance they'll remember and/or contact you when they need a website in the future?
Usability breeds familiarity which leads to a positive user experience. If there were two websites that contained exactly the same information and graphics, the website that was the most usable would win more sales every time.
Usability Online
The difficult thing with usability is that it's subjective - you might think a horizontal menu on a web site is easy to use, while we might prefer a vertical one - so you have to aim to make something usable for the overwhelming majority of people, because no matter what you do you'll never please everyone.
Before starting the design of a web site or piece of software, you should look at examples that exist in the real world. If you're about to design a Windows application then look at the usability of popular Windows applications such as Skype, MSN messenger, Outlook, etc.
Which design elements and objects make these pieces of software usable? Why? If you make a list of 10-20 things you'll have a great foundation for your own Windows application.
It's similar for web sites. If a client wants you to build a complete eCommerce website from scratch, then spend a few days (yes DAYS) getting intimate with Amazon, Shopping.com, eBay, etc.
Critique these sites from head to toe and come up with a list of features or strategies that you think make them usable. You can even copy a lot of usability features from sites like these, including:
The layout of menus
The positioning of images relative to surrounding text
Link placement and colors
Event messages, such as login and welcome text
Form layout and function
Color combinations and button design
Companies like Amazon have spent millions of dollars on usability testing, so doesn't it makes sense to take what's working for them and make it work for you?
Usability is the number one factor when designing anything, especially software or web sites. In most industries the product with the best design or ease of use will be the most profitable, because people don't have the time or patience to learn how to use a new product; they want something that's familiar and doesn't require a user manual to get started.
