What he said and... Its basically the new wave of websites that are very popular, they all have heavy focus on community and interactivity. Their names are often funky and leave out vowels and stuff. They do run mostly with DHTML effects and Ajax.
Basically its a style of website, with no technical difference.
What everyone has said is true. But if you ignore the gimmicky "Web 2.0" phrase and flashy front-ends, there's actually quite an interesting concept behind what Web 2.0 actually is, and how it's changed software development.