What is the difference between value types and reference types?

Value types store their data directly and are copied on assignment. Reference types store a reference to heap data, and copying copies the reference.

• Value types: numeric types, bool, char, enum, struct and tuples. Usually on the stack or inline in a containing object.
• Reference types: class, interface, delegate, string, arrays and records (unless declared record struct).

struct Point { public int X, Y; }
class Shape { public int X, Y; }

var p1 = new Point { X = 1 };
var p2 = p1;       // independent copy
p2.X = 99;         // p1.X stays 1

var s1 = new Shape { X = 1 };
var s2 = s1;       // same object
s2.X = 99;         // s1.X is now 99

Boxing wraps a value type in an object and is a common performance pitfall in tight loops. string is a reference type but behaves like a value because it is immutable. Prefer structs for small, short-lived, immutable data and classes for objects with identity.