"blog" is a term used for an online diary. It is shortened from "web log".
Blogs can be used for many different things and have many genres. Nowadays, people can get paid writing stuff on their blogs.
Besides blog, there's also "vlog" which is a video diary. You can find a lot of these on YouTube, where people make videos on their daily lives or talk about interesting things, etc. Also like the blog, you can make money by uploading vlogs on sites such as YouTube.