There was a lively discussion on one of the loops I belong to about what constitutes publishing. One writer listed a variety of mediums (including blogs, self-publishing, e-books, traditional print books, etc.) and asked if all of them qualified as a published credit.
I'm copying my answer below:
Publication has two definitions. In its broadest sense, it means dissemination of material to a wider audience.
In a stricter, more professional definition, it means being paid by a third party publisher (whether a newspaper, magazine, book publisher or e-publisher) for work as a writer. Self-publishing, individual blogs and letters to your mom do not fit this
Blogs are a form of self expression sometimes resulting in revenue. While some bloggers are paid by advertisers, the writers are not being paid to write. They are being paid to market. I blog as another means of marketing my written work.
While self-publishing is dissemination, it is not a professional credit because the author is the one paying to publish the work. Again, there may be revenue associated with the venture, but the author is the one who makes the decision that the work is worthy of publication. That is not an independent validation.
This is why most of the publishing world refuses to take self-publishing seriously. Although this is harsh, I once heard an agent describe it as the equivalent of a mother hanging a child's drawing on the refrigerator. Yes, the artist may be talented; and, yes, the mother may be proud, but it is not a professional credit.