In the context of the internet, a lifestream is a time-ordered stream of documents/posts/comments/pictures/videos that will function as a diary of your electronic life.
Every document you create, every photo or video you post and every comment, twitter or blog post you make is stored in your electronic lifestream - last ones shown first. Your past is at the end of your lifestream.
Some people even take it as far as to start off with a copy of their electronic birth certificate.
Lifestream entries: 12 Sep 20xx Posted entry on blah example blog
. 13 Sep 20xx Uploaded photo of my new cat 'Bert' to blah example photo sharing site. 13 Sep 20xx Twittered "OMG it is raining again"