Synchronous communication is interaction between people that occurs in real time, like chatting. Such communication is more engaging, but harder to moderate and organize.
Asynchronous communication means that people who participate in a conversation do not expect all the other parties to be in the same place at the same time and respond immediately. When communicating asynchronously, people have time to think, structure ideas, and format their messages. The resulting content usually has higher quality than in case of synchronous communication.
Communication in any of the three places can be synchronous or asynchronous, depending on the design of your community.
This is a fragment of a draft of the book “Lessons Learned While Working On Stack Overflow”. Read the full book on kindle or the paperback version.