Hi newcomer and welcome to the Tom's hardware forum.
I will assume that u are talking about the i7 (quad core) and the i3 (dual core), because doesn't exist any i7 dual core.
1- Performance: The i7 is a quad core processor with L3 cache and the i3 is a dual core processor with a high L2 cache BTU without L3 cache.
2- Price: The i3 star at $115 and the i7 star at $280.
3- RAM support: The i3 only support dual-channel kits and the i7-9xx series support tri-channel kits.
4- Components: Both the i3 and i7-9xx series works in different socket, 1156 for the i3 and 1366 for the i7-9xx series that means different chipset and mobos.