Is it a Diary? An accumulation of day-to-day events, documented for posterity?
Is it a Journal? Somewhere to keep secret thoughts, aspirations and disappointments?
Is it a Catalogue? Of websites visited, or interesting items spotted in various media which provoke further discussion?
Is is a Dissemination Tool? Something that can be used to spread information or knowledge about a specific subject or interest?
Is it a Money-making Machine? Somewhere to 'monetize' other people's browsing habits? Our American cousins and Web 2.0 gurus would have you believe so, albeit at the expense of a lot of hard graft and midnight oil.
Of course, the answer is that a blog is any or all of these, and more.
What will this blog be, I wonder?
