Code No. 1 = Arduino + UART

String command = "";

void setup() {
  Serial.begin(9600);  // Initialize UART at 9600 baud
  pinMode(13, OUTPUT);  // Set pin 13 as an output (LED)
  Serial.println("Send 'ON' or 'OFF' to control the LED");
}

void loop() {
  if (Serial.available()) {  // Check if data is available
    char c = Serial.read();  // Read a character from Serial
    if (c == '\n') {  // Check for newline
      command.trim();  // Clean up the command
      if (command == "ON") {
        digitalWrite(13, HIGH);  // Turn the LED on
        Serial.println("LED is ON");
      } else if (command == "OFF") {
        digitalWrite(13, LOW);   // Turn the LED off
        Serial.println("LED is OFF");
      } else {
        Serial.println("Unknown command");
      }
      command = "";  // Clear command for next input
    } else {
      command += c;  // Add character to the command
    }
  }
}

Practice makes perfect.

Serial.begin(9600);
This line initializes the UART at 9600 baud rate.

pinMode(13, OUTPUT);
Here we are setting a pin as the output, which we picked up the LED pin which is pin 13.

Serial.println("")
This line prints inside the terminal or arduino's interface

if (Serial.available()) {}
Why we use this line? Because we must be sure the user typed any message (ON or OFF), otherwise it won't function.

char c = Serial.read()
After checking it's available, read the data and save inside the c character

if (c == '\n') {  // Check for newline
      command.trim();  // Clean up the command
      if (command == "ON") {
        digitalWrite(13, HIGH);  // Turn the LED on
        Serial.println("LED is ON");
      } else if (command == "OFF") {
        digitalWrite(13, LOW);   // Turn the LED off
        Serial.println("LED is OFF");
      } else {
        Serial.println("Unknown command");
      }
      command = "";  // Clear command for next input
    } else {
      command += c;  // Add character to the command
    }

Here is the core of the work. which is obvious except like 
if (c == '\n') {  
      command.trim();

which is about checking and cleaning the line for typing inside the terminal

and command += c;

which is for typing more commands, not just one command