In computer programming, when a piece of data is passed to a function or method, there are two main ways in which it can be passed: pass by value and pass by reference.

Pass by Value

In pass by value, the value of the argument is passed to the function or method. This means that if the function or method modifies the value of the argument, the original value of the argument is not changed. This is because the function or method is working with a copy of the original value, rather than the original value itself.

Here is an example of pass by value in C++:

#include <iostream>
using namespace std;

void increment(int x) {
    x++;
}

int main() {
    int y = 5;
    increment(y);
    cout << y << endl; // Outputs 5
    return 0;
}

In this example, the increment function takes an int argument x and increments it by 1. However, when we call the increment function with the value of y, the value of y is not modified because the function is working with a copy of the value of y.

Here is an example of pass by value in Python:

def increment(x):
    x += 1

y = 5
increment(y)
print(y)  # Outputs 5

And here is an example of pass by value in JavaScript:

function increment(x) {
    x++;
}

let y = 5;
increment(y);
console.log(y);  // Outputs 5

Pass by Reference

In pass by reference, the reference (or memory address) of the argument is passed to the function or method. This means that if the function or method modifies the value of the argument, the original value of the argument is also changed. This is because the function or method is working with the original value itself, rather than a copy of the value.

Here is an example of pass by reference in C++:

#include <iostream>
using namespace std;

void increment(int &x) {
    x++;
}

int main() {
    int y = 5;
    increment(y);
    cout << y << endl; // Outputs 6
    return 0;
}

In this example, the increment function takes an int argument x and increments it by 1. When we call the increment function with the value of y, the value of y is modified because the function is working with the original value of y, rather than a copy.

Here is an example of pass by reference in Python:

def increment(x):
    x[0] += 1

y = [5]
increment(y)
print(y)  # Outputs [6]

And here is an example of pass by reference in JavaScript:

function increment(x) {
    x++;
}

let y = 5;
increment(y);
console.log(y);  // Outputs 6

It's important to note that in some languages, such as Python and JavaScript, all arguments are passed by reference.