DRM is the method e-book providers use to prevent copying, sharing, or to limit the amount of time you can download a book. It is different from the file format and set by the site where you download the e-book.
Different e-book providers use different DRM schemes which can limit which device you can use.
Common DRM schemes:
PDF files can be read on most any device. However, many e-book providers add Adobe e-book DRM restrictions to the PDF files which require you to download Adobe Digital Editions (ADE) or an ADE compatible app like BlueFire Reader.
Amazon e-books use the AZW file format and Amazon's DRM. You must use a Kindle device to read it.
Amazon offers Kindle apps for most tablets and smartphones and a Cloud Reader to read e-books from your PC or Mac.
ePub is a free and open e-book standard format. It is compatible with most eReaders and tablets with the exception of the Amazon Kindle.