What is Web site Usability?
Web usability is about designing your web site in such a way that your viewer can find what they're looking for quickly and efficiently.Finding your site is critical but, once found, the site must be easy to navigate and the content must be understandable. Without this the site is pointless.
When visitors reach your site, they want the experience to be a pleasant one. From the speed with which it takes to load the site contents (research shows that if a site takes over 6.8 seconds to load, the viewer goes looking elsewhere for your product or service), to the ease of navigation and the readability and quality of the text and the images used.
Never use frames or framing to lay out your web site .Frames can cause a number of usability problems, namely:- Disabling the back button in browsers.
- Search engine optimisation issues.
- Bookmarking is not possible.
- Impossible to e-mail the link to someone else.
- Problems with printing the desired web site page.
- Users are turned off by links which do not open, or are slow to do so.
A good website is an easy to use website.

Usability Analysis of Apple.com
One of an important thing of web design is web usability that make your website easy to navigate, fast access, easy to read and users are not confuse. Apple is a good one that they have a good usability website (apple.com). I found this article on Digg, it’s all about how Apple design a good website and good web usability on their official website.
Any website (or any design layout for that matter) starts out as a simple grid. Grids are the foundation to any layout and the content is built upon that grid. Apple uses very clean and well-spaced grids on each page, which makes for a very pleasing experience.
On top of a smooth layout, the colors also compliment the usability of the site. Color schemes are crucial to the feel and vibe of the website. Very often coloring does effect usability of a site, believe it or not.