a stream of content from a variety of services in one location so your friends/followers can visit one site and see which photos you uploaded, what your Twitter updates are, your latest blog posts and more.
Like a lot of geeky professionals I use Twitter. I also use the social bookmarking site Ma.gnolia, photo sharing site Flickr, and Pownce. Every input into one of these services is fed into Tumblr ( see Jack of All Trades ) and Plaxo Pulse. I don't use Facebook for my life stream because it would require anyone wanting to stalk me to register an account.