A Kindle is an Amazon reading device that can display both Kindle books and PDF documents. Kindle books can be purchased from Amazon, or checked out of the library.
A Kindle Fire is an Android tablet, which is not the same as the other Kindle models.
Kindle for Beginners:
How to use a Kindle,
find new books, and
check out a book from the library.
How to email a document such as a PDF to your Kindle.
More sources for books.
Changing reading settings.