In tech circles, vanilla means the simplest possible version of a program or web site. A vanilla version of a web site would have no bells and whistles - only the essential components that it needs to function.
Vector graphics differ from bitmap or raster graphics in that they are not made up of a grid of pixels. They rely on computer algorithms to define paths and shapes. Vector graphics are rarely used in web design, but are very common (required) in graphic design. A logo, for example, would be created in vector format. From there it is exported to bitmap/raster format for use on a web site.
The main advantage is that vector graphics are fully scalable. You can print your vector logo the size of Texas without any pixelation along the edges.
A form of marketing where the user of the product or service helps to spread the word. The birth of the Web meant that ideas could spread quickly from one person to the next. Clever marketers turned that ability of the Web into one of the most powerful marketing techniques ever.
A great example is that of Hotmail. The early success of Hotmail is largely down to a small "Get your private, free email at http://www.hotmail.com" message at the bottom of each email sent from Hotmail. The more users they added, the more "Get your..." messages were being sent out into the world.
The amount of time that any particular visitor spends on a site. Webmasters are usually not interested in individual visit durations, but rather in the average visit duration for all visitors to a site. Analysing visit duration is tricky. If the average visit duration is short, it could either mean that the average visitor does not find the information they come looking for, or it could mean that they find it very quickly. The type of site and the type of information offered must be taken into account if visit duration is measured.
Short for vertical portal. It's a type of web site that functions much like a traditional portal, but the content has a much sharper focus. Typically a vortal would focus on a specific topic (e.g. dogs) or a specific industry (e.g. web design).