>>50
From a technical point of view, dynamically modifying webpages based on a server-side application's response to user input was made possible by the XMLHttpRequest JavaScript API, which was implemented started in mid-1999 in IE and late 2000 in Mozilla (though in both browsers it wasn't particularly mature until roughly 2002, and not a completely formal specification until 2006.)

"Web 2.0" as a term also was popularized by a conference in 2004 though it first appeared in someone's article describing the future of the internet in 1999.

I think the first widely used Web 2.0 application was Gmail in 2004 followed by Google Maps in 2005. There were earlier websites that made use of dynamic webpages but none became all that popular, or survived for very long (except for Microsoft's shit.)

>>48-51
I think the terms are a bit nebulous and depend on who you ask. Some people think HTML webpages with cgi-bin scripts are the beginning of web 2.0, and those sites date back to the late 90s. I think for most people though, web 2.0 is a design philosophy. The era of round corners, gradients, icons, user interaction, and so on. Social media is probably the biggest delineating point, but then again, blogs and forums and 2ch style BBSes fall into that category as well.

It's just one of those things that you know when you see it. The details are a bit murky though.