lvalue, rvalue

lvalue(locator value): Anything that can hold a address. Appear on the left-hand side of an assignment.
rvalue(read value): temporary value that does not occupy a persistent memory location. Appears on the right-hand side of an assignment

int a = 10;                 // a is l value
                            // 10 is rvalue
int x = a+b;                // x is l value
                            // a+b is rvalue
        

lvalue reference or Reference Variable (&)

Reference to bind to lvalue. This is same as reference variable
Usage: Pass large objects without copying.

int a = 10;
int &b = a;         // lvalue reference
b = 11;
cout << a;    // 11
        

Example: Copy Constructor is lvalue reference

rvalue reference (&&)

Reference that binds to an rvalue(temporary value).
Usage:
  Move semantics (ie moving a temporary withut copying it).
  Perfect forwarding in template functions.

int a = 10;
int &&b = 10;            // rvalue reference
        

Example: Move constructor

XValue / Expiring Value

(b+c) is called xvalue/temporary value which is ready to expire

a = b+c;  
        

PRValue / Pure R Value

Literal value is called pure R value

a = 42;