Adobe Reader, reads PDF files which are created by Adobe Acrobat.
The beauty of this, is that Adobe Acrobat can convert almost file type from the popular Operating Systems. That means a document can be created by almost any program and then given to another person to read, even though the receiver of the document doesn’t have the program that created the document.
For example, a writer can create a Microsoft Word document on a Windows machine, convert it into a PDF file and give it to a person using a Linux machine and they can use Adobe Reader to read it. The Linux user doesn’t have to have Microsoft Word to read the document.
The Reader is free, but Acrobat costs money.