In a little over 2 years after the initial beta testing, Zwift has grown to become probably the biggest player in indoor community cycling. Rather than write here what it's all about, I'll simply link you to their own website where you can learn yourself what a stunning experience it is for us indoor cyclists.
For me personally, after a winter of using Zwift I regained much of my previous fitness, which given how ill I was on my arrival here in the Netherlands is nothing short of remarkable. I have become heavily involved in Zwift, both from a team standpoint with Team WBR but also from a community aspect with my duties as a regular Zwift ride leader. It has become my prime means of exercising during the poor weather that is all too frequent here, but it's much more than that to me as I have made many friends through the platform.
Here's a good promotional video, or click on any of the Zwift links to go direct to their website.