Weblogs, often referred to as blogs, are not a recent invention, but they have been generating a lot of interest lately. Weblogs are used for so many purposes these days it is difficult to define exactly what one is. However in basic terms they are really just web sites that are updated on a regular basis with human content, almost like an online diary. They are very commonly kept for entirely personal reasons and a typical blog might simply contain posts about the author's life or links to interesting websites they have come across. Blogs are not always kept by just one person.
Group blogs are nearly as common.
There is also a growing number of commercial blogs, such as the popular tech forum SlashDot.org. Having brought popularity into the equation, I should note that attempting to become the next SalshDot for your industry should not be your sole motive for starting a blog. A successful blog doesn't have to be a big blog. In this and many other cases the mix of business and personal has really worked. Obviously it is not always appropriate to combine the two, so consider your individual situation and set aside some ground rules first.