Simply put self-publishing is when you as the writer of said book, also happens to be the chief market personnel and distributor and the person who spends the money.
So, this means that to most writers they will be doing far more work than they would like to think that they should (or want) to do. This does not mean that we should think that we do not need to self-publish from time to time, but we need to look at how to promote and encourage self-publishers.
There are many ways that even writers who traditionally publish can also be considered self-publishers. This is not a negative thing or some thing that writers (in particular those who would never think to self-publish) need to consider.
Writing and publishing a blog is self-publishing. this does not mean that every blog writer will make money through their blog, but with the correct promotion and the will to write a blog on a regular basis this will mean that they have the potential to make money. The logic that is used to say that you are a self-publisher when you write and publish a blog is this: You are not paid by a third party to do it, and you generally are passionate about what you do.
Self-publishing is also when you have done a small “booklet” or some thing voluntarily. Often this is done for a community service, and although you will get credit for your work, often once the booklet is done, if you want to market it, you will need to do more to sell it. This is not of genuine self publishing, but more some thing you can do to make a bit of money with your writing.
Self-publishing does have a place in the world of writing, as does traditional publishing, but to dismiss one or both is not some thing that writers should do without understanding what it means to publish.