The state of a web site when it is not finished yet, but design work has started on the site.
Having users navigate to an "under construction" page wastes their time so it is considered bad form. The alternative is to have a lightweight version of the site as a place holder while the full site is being developed.
Usability refers to how easily web site visitors can find what they are looking for or accomplish a task such as contacting the company. The study of usability is critical to e-commerce, where the aim is to make the customer's path to the order page or sign-up page as clear as possible.
Web site content that is generated by the people using the site, rather than the owner(s) of the site. This web site is a good example. Visitors to WebDesignDictionary.com like you and me add and edit terms and definitions. At sites like this one, the owner's role is usually that of editor/moderator, ensuring that the content uploaded conforms to site policies.
Unique Selling Proposition. It is generally accepted that, for online sales to happen, a site must have a USP. In other words, there must be a reason for people to buy from that site, rather than from its competitor. GoDaddy.com is not the only domain registrar, but they offer domains comparatively cheaply, they have pretty girls on the homepage ;) and they have a fancy domain management area for their customers. Any or all of these can be considered the GoDaddy USP.