A blog is the equivalent of an online journal and there are now blogs about every topic you could imagine.  Blogs are usually started by individuals passionate about a particular topic or by businesses wanting to build their brand profile and showcase their industry expertise.  You can add links to other sites and insert videos, photos, articles etc.

The main benefit of a blog is that it is a very low cost way to get online quickly & easily.

The two primary blogsites were you can site up for a free site are:
www.wordpress.com
www.blogspot.com

If you sign up for a free site your URL or domain name will be something like mysite.wordpress.com
If you choose you can upgrade your blog to a paid account which gives you access to premium features.

It is possible to have a blog integrated into your website, exactly how this is done and the cost will depend on the provider you choose.  Blogs on sites are great for SEO as they are an easy way to keep adding new, regular, relevant content to your website which will help to improve your search engine rankings.