Structure is value type whereas class is reference type.

Structures have two performance advantages over reference types. First, if a structure is allocated on the stack (i.e., it is not contained within a reference type), access to the structure and its data is somewhat faster than access to a reference type on the heap. Reference type objects must follow their reference, or pointer, onto the heap in order to get at their data. However, this performance advantage pales in comparison to the second performance advantage of structures: namely, that cleaning up the memory allocated to a structure on the stack requires a simple change of the address to which the stack pointer points, which is done at the return of a method call. This call is extremely fast compared to allowing the garbage collector to automatically clean up reference types for you in the managed heap.