A blog is a modern communication vehicle that is becoming more and more important in today's Web-connected world.
/rant on
A blog is not TrackBacks, pings, Digg It buttons, blog rolls, permalinks, or Google ad sense. You will see all of these things on a blog (hopefully not all at once), but these are just features of a blog. You can have a successful blog, even the top 50 heavyweight, without any of these. As you create your blog, make sure you don't let the trees get in the way of seeing the great potential of the woods.
I would not go so far as to say that there are not benefits to be found in social bookmarking sites such as Digg or del.icio.us, but I don't think it's as critical as people make it out to be - for several reasons.