Answer:
They are toxic substances that cause cancer, they do not cause it right away they take a while to cause cancer depending on exposure.
Certain forms of radiation are also classified as carcinogens, for example radioactivity, sunshine and electric fields. Exposure, even long term, to carcinogens will not necessarily cause cancer (and doesn't!) but theoretically just one molecule of a carcinogen can ('no safe dose').
The IARC (International Agency for Research on Cancer) lists nearly 1,000 carcinogens, including yesterday's addition of mobile phones.
Ozzy Osbourne said something like, "If you let your life be affected by the worry of something that may or may not kill you in fifty years' time, you're already dead." But then Ozzy isn't exactly the ideal role model!