If you're going to start a blog, you're going to have to write (unless you're video-blogging, of course).  Majority of folks, though, will be keeping blogs in written form and there are three major approaches you can take.

As A Hobby

Most people start up blogs as a hobby, using it as a platform to talk about their interests and pastimes.  Since they're done more out of passion than any commercial interest, hobby blogs aren't typically the source of great writing.

For Promotional Purposes

Many businesses run blogs to promote their products and services.  In a similar way, some part-timers use them as a platform to sell affiliate products as a way to make extra income.  Generally, these blogs are professionally-written, properly edited and filled with qualified information.  After all, sloppy work is going to make it pretty tough to convince prospects.

As A Profession

Professional blogs, put simply, are designed to make money directly from the medium.  Rather than promote a product or amuse the writer, their main goal is to provide a source of income all on their own.  Sure, many professional bloggers crossover into promotional territory, but a professional blog is rather easy to tell, as it earns its own keep, either via advertising or some other means of generating money.

Personally, I prefer professional blogs that are well-written, properly edited and with clear language.  In fact, I implore everyone putting together their own to invest in a writing software and go that route.  It's not mandatory, though.