This document discusses the characteristics and structures of online communities. It defines online communities as groups of people who come together online for a specific purpose. It describes how social networks form the underlying structure of communities, with nodes (people/organizations) connected by relationships and experiencing interactions. It outlines key characteristics of online communities like conversations, presence, democracy, standards of behavior, and levels of participation. It also discusses how ideas and memes spread through communities via opinion leaders and the two-step flow model of communication.