There are several types of CAPTCHAs used to verify human users. Text-based CAPTCHAs present distorted letters and numbers that users must type correctly. Image-based CAPTCHAs require users to identify specific objects or patterns in images. Audio CAPTCHAs provide a sound clip of distorted audio that the user must transcribe. More advanced CAPTCHAs, like Google's reCAPTCHA, analyze user behavior on a webpage to determine if they are human without requiring active problem solving.