Web Challenges are several technologies used to distinguish between real humans and bots in general, or DDoS bots in our context. There are several types of challenges. The best-known one is the CAPTCHA, which is very intrusive, but there are other, less-intrusive transparent challenges such as Cookie Validation or the JavaScript Challenge. Web Challenges are considered one of the most effective mitigation layers against DDoS application attacks.
Related entries: Cookie Validation, JavaScript Challenge, Web Challenge Spectrum