Well, a .sparseimage is a file that can be mounted like any other disk, and will 'grow' based on the amount of data stored in it. So, a 30GB .sparseimage, when empty, may only be a few hundred K. As you place more data into it, it gets larger, until it reaches the maximum size set when it was created.
A .dmg is another kind of disk image. It's of a fixed size, and can be either read-only or read-write.
Page 20 of the manual goes into some detail about what SuperDuper! does when it
directly creates an image -- that always results in a DMG, with a .sparseimage as an intermediate step. I recommend reading that page for additional ifnormation.
However, as described in
this post, you can use a sparse image directly to store backups in a "single file".
Hope that makes it clearer!