I am probably gonna butcher this a lil but I am gonna give it my best shot. From what I understand one of the main disagreements is how you connect with god. From the sunni perspective you can communicate directly through prayer. God will hear you personally. The Shia often communicate through a mediator who kinda fast tracks their prayer to god. I think these mediaters are usually Imams, even dead ones but I am stretching my knowledge a bit here.

The way I had it described to me kinda reminded me of protestants and catholics with how catholics speak to god through a priest. Would love to know how correct I am here because its been a while since I studied this.

Edit: oh yeah there is also some saint veneration of Imams that sunnis consider idolatry