Where Self-plagiarism Begins and Where it Ends
Let us set aside debate about whether the word "self-plagiarism" makes logical sense. The concept is fairly generally acknowledged, and rests on a simple ethical principle: it is wrong to give the impression that a work is novel when it is not.

We can further distinguish two distinct forms of self-plagiarism:


Literal self-plagiarism, in which words are directly reused.
Conceptual self-plagiarism, in which the words are different but the content is (largely) the same.


From this definition, we can elaborate several key points of evaluating whether a reuse counts as self-plagiarism:


Is there an expectation of novelty (e.g., a journal article, conference paper, or book)?  In some cases, such as one-paragraph conference abstracts in some fields, there may not be an expectation of novelty, in which case there is no "impression of novelty" to be considered and self-plagiarism is not at issue (the reviewers may still decide they are uninterested if the work is insufficiently novel).
Is more than a trivial amount of material being directly reused?  If so, then they must either be in a quotation, or else the publication must be clear that portions have previously been published, and a citation to the prior publication must be included.  Otherwise, it is self-plagiarism. (Note that this may not apply to "non-creative" words, such as a tersely written methods section).
If concepts are shared with prior publication, then there must be clear attribution of the significant antecedents.  There is a lot of flexibility that enters with "significant," and different communities interpret this in different ways, so you need to know your community expectations.  Here, one basically treats one's own results just like those of any other scientist, with the exception that one cannot claim ignorance of one's own prior publications.