1- Captcha which could be automated-
This is the example from- http://www.indianrail.gov.in/pnr_Enq.html

import java.util.concurrent.TimeUnit;
import org.openqa.selenium.By;
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.firefox.FirefoxDriver;
public class CaptchaAutomated {
public static void main(String[] args) {
WebDriver driver = new FirefoxDriver();
driver.manage().timeouts().implicitlyWait(10, TimeUnit.SECONDS);
driver.get("http://www.indianrail.gov.in/pnr_Enq.html");
String captchaValue = driver.findElement(By.xpath("//span[@id='txtCaptchaDiv']")).getText(); //this will read the captcha
System.out.println(captchaValue);
driver.findElement(By.xpath("//input[@id='txtInput']")).sendKeys(captchaValue);
}
}
Output- This program will read the captcha and will enter the value of captcha in the Enter Captcha box.

2- Captcha which could not be automated-
This is an example from - https://signup.live.com/signup.aspx
Such kind of captcha can not be automated.

This is the example from- http://www.indianrail.gov.in/pnr_Enq.html
import java.util.concurrent.TimeUnit;
import org.openqa.selenium.By;
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.firefox.FirefoxDriver;
public class CaptchaAutomated {
public static void main(String[] args) {
WebDriver driver = new FirefoxDriver();
driver.manage().timeouts().implicitlyWait(10, TimeUnit.SECONDS);
driver.get("http://www.indianrail.gov.in/pnr_Enq.html");
String captchaValue = driver.findElement(By.xpath("//span[@id='txtCaptchaDiv']")).getText(); //this will read the captcha
System.out.println(captchaValue);
driver.findElement(By.xpath("//input[@id='txtInput']")).sendKeys(captchaValue);
}
}
Output- This program will read the captcha and will enter the value of captcha in the Enter Captcha box.
2- Captcha which could not be automated-
This is an example from - https://signup.live.com/signup.aspx
Such kind of captcha can not be automated.