Both ISBN and ISSN are unique product identifiers. They come in a series of numbers and not one product has the same code as the others.
ISBN (International Standard Book Number) identifies individual book for its language, title code and its publisher. It is used to distribute and order your title at product sellers across the world.
ISSN (International Standard Number) identifies the series of books published under a unifying title but does not identify the publisher. Although ISSN is usually applicable to magazines, journals, newspapers among others, but it is not really limited to periodicals; they can also be applicable in some book series. The same ISSN code is used all throughout a particular series.
Can go either or both, ISBN and ISSN are located at the back cover and inside the copyright’s page.