Adobe changed their naming convention with the release of their 6-line of Acrobat products.
Earlier, there used to be Adobe Acrobat 5 (commercial) and Adobe Acrobat Reader 5 (free).
Now, Adobe Acrobat Reader is just called Adobe Reader. Adobe Reader 6 is the free version that lets you view PDF files and fill out forms that have already been created. Adobe Acrobat 6 (Standard and Professional), is the commercial version of the software that you have to pay for. It not only allows you to view PDF files but also lets to edit them, create PDF forms, and convert other documents to PDF files.