Difference between iPad 8 and 9

Both are the entry-level iPad - the iPad9 being a year newer. You can directly compare all models of iPad here:

iPad - Compare Models - Apple

The most significant technical benefits of the iPad9 are an updated CPU (now the A13 CPU instead of the A12) and a 12MP ultra-wide angle front camera - that includes CentreStage FaceTime camera support - that replaces the previous 1.2MP camera.