what is the difference?

First off: Itch does only work with Itch certified hardware, period. The hardware manufacturer pays a license fee to serato for itch to work.

The limitations are quite simple: Only 2 Decks, less effects, less samplers and what you can do with them, less hotcues, different style, less options and tuning, different beatsyncing (both work) and some fancy features are missing. No expansion capabilities. For a full list I would check out the webpage of serato and look at both product specs.