A CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) is a type of challenge-response test used to determine whether the user is human or a computer. It is commonly used on websites to prevent automated bots from performing actions such as creating fake accounts, sending spam, or abusing online services.

CAPTCHAs typically present puzzles that are easy for humans to solve but difficult for machines. These puzzles can take several forms, including:

1. Text-based CAPTCHAs: These display distorted characters or words that users need to identify and type into a text box. The distortion makes it hard for bots to read the text using automated algorithms.

2. Image-based CAPTCHAs: Users may be asked to identify certain objects in a set of images, like selecting all images with traffic lights or street signs.

3. Checkbox CAPTCHAs: These are simpler "I am not a robot" checkboxes, where the system uses behavioral analysis to assess whether the user is likely human based on their interaction patterns (e.g., mouse movements).

4. Audio CAPTCHAs: For visually impaired users, audio challenges may present distorted speech or sounds that the user needs to interpret.

CAPTCHAs are effective in preventing automated programs (bots) from interacting with websites, ensuring that actions like logging in, submitting forms, or making purchases are done by legitimate users.