Zip Disks are a bit thicker than a 3.5" floppy disk and can store a lot more information.
Disassembled zip disk
The inside of a zip disk is very similar to a 3.5" floppy disk. The main difference is that the circular disk is a bit larger in diameter.
Zip drive
One downside to zip disks is that they required a special zip disk reader, which weren't universally available. This zip drive is from the WCSU archives.