I have both of those devices on my desk. The originals of both. The similarities of both: rectangular, displays, batteries (dead), audio jack, charging ports and could play music.

Aside from the large list of physical differences (like, one has cameras while one has a big flat physical turntable as its input device), the biggest category of differences is as large as the number of unique apps there are in the app store. Because the biggest difference is: one does anything, the other does one thing.

Still, you're right. One doesn't leap over oceans of differences. There were many products and ideas between the original iPod and the iPad, each one potentially sharing with or influencing the other.