The Long Tail - The idea is that the web due to its ability to reach so many people means that collectively niche markets can be more powerful than 'the mainstream'. Chris Anderson wrote an article about it in Wired magazine and although some of his maths has been questioned the principle is sound enough.
So it may be 10,000 bands on MySpace selling a million albums between them or a record company selling the same amount by one artist or the mass of blogs versus conventional media.
Items that are in low demand ie have a low sales volume can collectively have a market share bigger than best sellers or blockbusters, this is used to explain the power of user-generated content.
Actually harnessing user-generated content isn't new: AOL was built on user generated content: emails, chat forums and IM conversations.
Context is used to explain the success of products because they fit into peoples online lives: for example Wikipedia and Flickr are simple ideas well executed, however they are powerful because people understand clearly what they are and what they do.
Context is also used to describe the increased level of interpretation that can be put on data when meta data like user generated tags are used, or the data is complemented by other relevant information from different sources (mash-ups in web parlance) like 118118.com do with their directory database and Google Maps so you can actually see where the restaurant is that you want to go to.
From a PR perspective, context is what you need to provide in your PR programmes for your Web 2.0 client, often it can be quite hard to articulate how their service is relevant to the average consumer's online life and why it is beneficial of them to take the time to tag?
There is a school of thought in product design that goes something along the lines of if a product isn't intuitive enough to be used without providing how-to guidelines its badly designed, what can I say most of your clients will fall into this technically talentless category.
The Web as Platform - the web is not not only being used as a way of hosting websites but also as a way of delivering a service or application, this isn't a new concept in businesses, as Salesforce.com and MySAP have been doing this for years; but its a major change in the way consumers do things.
Instead of relying on that bootleg copy of Word from the IT manager at work, they and draft their CV using Writeboard and similar services, share their wedding photos via Flickr or help their children use eyespot to edit a three-minute happy slapping in the playground video recorded on their mobile phones.
In addition as they build up a critical mass of users, they start to become even more useful, think about the way that Last.fm recommends new music to you based on your personal ratings and playlists, how how eBay became the world's market once enough people started using it.
This is driven by a process called group-forming networks where users band together through altruism. There is a network effect where the value or power of the network grows exponentially as the numbers within it increase. Think of it as a pyramid scheme without the lots of people getting defrauded at the end bit (well apart from from the fund managers who are negligent in the way they invest for our retirement).
Ok, so an online word processor is not going to be as full featured as using one on your desktop machine because you're using it over a net collection and it would be tediously slow.
This means that many web applications like Backpack or Writeboard concentrate on doing one thing well with a minimum of fuss, providing a better user experience: making a technological silk purse out a pigs ear.
