The Captcha Type field set controls what kind of captcha will be used during account registration, password recovery, and if a user wants to suggest a url. The choices for captcha are:

• Hash captcha, the user's browser (the user doesn't need to do anything) needs to extend a random string with additional characters to get a string whose hash begins with a certain lead set of characters.
• Graphic Captcha, the user needs to enter a sequence of characters from a distorted image;

Of these, Hash Captcha is probably the least intrusive but requires Javascript and might run slowly on older browsers. The graphic captcha is probably the one people are most familiar with.