Frassle has a lot of different stuff in it. I'm searching for a short explanation that explains the unifying themes. Here's attempt #1:

Frassle is an interpersonal content management system—a connection point for conversations you're engaged in. It is designed to help you find relevant discource, participate in it, and share it with others.

All of this is made possible by the content of your blog and other blogs that hook into frassle via RSS. Frassle's aggregator helps you follow news sources you care about. Your blog collects thoughts (soon including private ones). Throughout, frassle threads each post in context—whether it's a comment, a response in a frassle blog, or a response aggregated from an external source. Finally, the frassle content studio lets you build dynamic web pages and RSS feeds that present content from sources you choose.

None of frassle's functions are brand new, but the coherent way that these functions are integrated is new.