What is a blog?

First and foremost, if you're going to start a weblog, you're certainly going to need to know what it is.

A blog (a portmanteau of web log) is a website where entries are written in chronological order and commonly displayed in reverse chronological order. "Blog" can also be used as a verb, meaning to maintain or add content to a blog.

Source: http://en.wikipedia.org/wiki/Blog

And that would pretty much cover a rough definition of the term "weblog" or "blog". Of course, that's not all there is to it. The substantial difference (as far as I can see) between a weblog and a website is the content: usually, weblogs contain much more commentary (opinions) as opposed to a website carrying much more information (facts).

A common misconception is that blogs are just online diaries and journals - and I agree, a majority of blogs are such. However, there are also a good chunk of blogs dedicated to political, technological, and entertainment commentary and such.

If you're in it for the money, I suggest you avoid the personal blog niche. Sure, it might work wonders for your mind and serve as great therapy, but no one really wants to read about complete stranger's lives (usually).

If you want to learn more about blogs, just do a simple Google search; they are growing exponentially much more popular both as a means to get your voice heard as well as a reliable way to earn money. If you want a little more substance right now, I suggest you take a look at ProBlogger.net, a site which I've found very informative.