What is difference between merlot and zinfandel?

They are different cultivars of grape. Cabernet Sauvignon is a hybrid formed centuries ago in Bordeaux by the cross pollination of Sauvignon Blanc and Cabernet Franc. Zinfandel is a misnomer of the variety Primitivo/Kriejenak Castlanski from Croatia.

Think of the difference between a border collie and a Labrador. Both are dogs but are bred in different places to enhance different characteristics.