A blog is a diary that you don't write in a notebook but on the Web.
Like in a diary, you can make many uses of it: you can tell about your daily life, about particular events, about politics... It's just that instead of keeping it private, a blog can be read by anyone. It is more lively than a notebook too because you can also publish pictures, videos or music on it.
But what you make of it is really up to you!
Hope this helps!