What is the purpose of the Cutting Guide Attachment?

The Cutting Guide Attachment is designed to help the user maintain depth while using Dremel spiral saw cutting accessories like the 560 (drywall), 561 ( Multipurpose), and 562 (ceramic wall tile).

What’s the difference between the Dremel the 7700, the 7300, and the 7000?

The key difference is that the 7700 (7.2 V) and 7300 (4.8 V) both come with rechargeable ni-cad battery packs. The 7000 works on alkaline batteries that you supply.