What is the difference between online and offline communities?

Good question and from my perspective the main differences are these:
- Offline communities are most often bounded geographically, even if vaguely
- The process of communicating and building relationships online is different than offline. It takes a bit longer for deep connections to form but faster for weak connection to form and there is what I call digital body language instead of physical body language and it takes a while to understand and identify.
- Online, you may still not get the whole picture of someone because they they can still (unless FB has its way) be relatively anonymous and you see them in a very discrete context whereas offline you are more likely to run into them in the grocery store, etc.
In many other respects, they are the same.
Hope that helps -
Rachel