RSS (Real [or really] Simple Syndication is a technology that enables a user/customer/client/interested party to subscribe directly to a feed (a file of of constantly updated information) from the publisher source.
RSS is not really a new development, it's been around for a few years, but only the past 24 months or so has it managed to gain a wider adoption (I would not neccessarily say that it's widely adopted yet though).
The problem as I see it (RSS) is that most end-users/consumers are still unaware of what it is and how to use it and hence don't have the level of interest that in my personal opinion it warrants. Over time, most consumers have mentally defaulted to email as their primary means of communication.
RSS provides a better and more real-time experience, but as with all great technologies, just because it's better, faster, cheaper, etc., does not mean that it won't take ten years or so to reach mass adoption.
Internet Explorer included an RSS feed reader in their most recent browser update, so RSS has some potential if the Web industry as a whole can explain the benefits of using it over email.