The context of a post is the series of posts that led up to it.
A thread, on the other hand, is all the replies to a post, and their replies, etc. Click on n replies to get the thread starting at a post.
A whole thread is a thread starting with a head post; a head post is any post that isn't a reply.
Click on a user name to see that user's blog. Click on a message id to view just that message.
Clicking reply to this lets you to reply to a message.
A very short answer would be: a better Twitter.
A longer and more accurate answer: a new kind of internet conversation platform.
Right now, it combines the best features of Twitter, blogging, Usenet and bulletin boards.
Later it will include private messaging features, similar to email or facebook.