The ISSN (International Standard Serial Number) refers to the title of a serial publication, that could be a journal, magazine but also a book series. The ISSN stays the same from issue to issue until the title of the series changes.
The ISBN (International Standard Book Number) refers to a single, specific volume such as a book, novel or a monograph.
Normally a book has only and ISBN number, but when it is part of a series it has an ISSN as well.