What is "bad traffic"?
First of all we have to take a look on "good traffic":
"Good traffic" is Internet traffic to a website or a web service, which can be monetized by advertisement or in some other ways. In this case the webmaster earns enough to pay for the server/hosting and gets perhaps some extra money for himself.
"Bad traffic" is Internet traffic to a website or other web services, that create a huge amount of traffic and by this, costs to the webmaster. The difference between "good" and "bad traffic" is, that webmasters have big problems to monetize "bad traffic". Because they get really small money for the clicks on advertisement.
Bad traffic is often abusive. Lets take a look on an example: web proxies (http://mytwitterproxy.com) get much abusive traffic from the Islamic Republic of Iran, Iraq or Saudi Arabia. Users from this countries often visit pornographic content and create massive traffic. When they click on advertisement, there clicks are mostly less worth than one US-cent. As you can see, the income does not cover the expanses.
We assume, that you can categorize good and bad traffic by their origin. In fact you can say that good traffic (good to monetize) comes from North America and from countries in Europe. Really "bad traffic" comes from the Near-East-Countries and from some Asian countries.
This is only one possible definition of good and bad traffic.
