Program code, often referred to simply as "code," is a set of instructions written in a specific programming language to instruct a computer on how to perform a task or solve a problem. 

Program code must be written using the correct syntax, otherwise the computer will not be able to interpret the code and execute the instructions.

Syntax rules are vital in programming, governing how code is structured for computers to understand. Similar to grammar in language, these rules ensure accurate communication between programmers and machines. Proper syntax is crucial for error-free, readable, and functional code, making it a cornerstone of effective programming.

Program code written with incorrect syntax will result in a syntax error.

Python is a high level interpreted language that is easy for beginners to read and write.

name = input("Enter your name: ")
print("Hello", name)

Java is a high-level programming language that is more difficult to learn than Python because it has stricter syntax and requires more code, but it is still relatively beginner-friendly.

import java.util.Scanner;
public class Main {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        System.out.print("Enter your name: ");
        String name = input.nextLine();
        System.out.println("Hello " + name);
    }
}

Assembly is a low-level programming language that is much more difficult to learn than Python and Java because it uses hardware-level instructions and requires a strong understanding of memory, registers, and the CPU.

; Input/output depends on the operating system.
; Example using Linux x86-64 system calls:
section .data
    msg db "Hello World", 10
    len equ $ - msg
section .text
    global _start
_start:
    mov rax, 1          ; sys_write
    mov rdi, 1          ; stdout
    mov rsi, msg        ; message
    mov rdx, len        ; length
    syscall
    mov rax, 60         ; sys_exit
    xor rdi, rdi
    syscall

When program code is executed it needs to be converted into binary machine code in order for the machine process the instructions. Depending on the programming language this is the job of either a translator, compiler or interpreter.

In order to compile the code it goes through a number of steps, which depending on the language might include:

1. Preprocessing
2. Compiling
3. Assembly
4. Linking.

Pros

• Executable and can produce desired results

• Allows precise control and manipulation of computer resources

• Provides a practical understanding of programming concepts

Cons

• Requires knowledge of programming languages

• May involve complex syntax and structures

• Difficult to understand for beginners without proper guidance