A CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) is a technique that allows to distinguish between human and automated requests. In VB.NET, a CAPTCHA can be implemented by generating a random text or image, displaying it to the user, and asking the user to input the text or characters displayed. Once the user inputs the text or characters, the application compares the user’s input with the original text or image, if the inputs match, the user is considered to be human, otherwise, the user’s input is considered to be incorrect.

For example, a text-based CAPTCHA may generate a random string of characters, such as “K2M3N”, and ask the user to input the characters displayed. If the user inputs “K2M3N”, the application will consider the user to be human, otherwise, the user’s input will be considered to be incorrect.

It is important to note that text-based CAPTCHA can be easily bypassed by computer programs, so it is recommended to use image-based CAPTCHA or reCAPTCHA which are harder to bypass and more secure. These types of captchas use images with distorted text, numbers or images and ask the user to identify them.

Another important aspect to take into account is that CAPTCHA should be designed in such a way that it is easy for humans to solve, but difficult for computers to solve. The CAPTCHA should also be designed to be accessible for users with disabilities.

Here’s an example of how you might create a text-based CAPTCHA in VB.NET using the Random class to generate a random string of characters:

' Create a new Random object
Dim rnd As New Random()

' Generate a random string of characters
Dim captchaText As String = ""
For i As Integer = 1 To 5
    captchaText &= Convert.ToChar(rnd.Next(65, 90))
Next

' Display the CAPTCHA text to the user
CaptchaLabel.Text = captchaText

' Compare the user's input to the original CAPTCHA text
If UserInputTextBox.Text = captchaText Then
    ' The user's input is correct
    MessageBox.Show("Correct! You're a human.")
Else
    ' The user's input is incorrect
    MessageBox.Show("Incorrect! Please try again.")
End If

This code generates a random string of 5 uppercase characters and displays them in a label control. Then it compares the user’s input to the original CAPTCHA text, if it matches the text, a message is displayed to the user indicating he is a human, otherwise it indicates that the input is incorrect.

Keep in mind that text-based CAPTCHA can be easily bypassed by computer programs, so it’s recommended to use image-based CAPTCHA or reCAPTCHA which is harder to bypass and more secure.