In the scheme of things, testicular cancer is one of the rarest cancers. BUT it's the most common cancer occurring in young men (that's probably your age group - 15 to 40 ish). It happens in about 2000 men per year in the UK. The good news is that it's one of the most curable cancers, especially if caught early.
Testicular cancer is often painless, so checking your testicles regularly is the most effective way of spotting anything that might concern you.
What's important is to know what's normal for you, so you should be checking your balls regularly. If you spot any change in the way your ball, balls or scrotum looks or feels then get an appointment with the doctor as soon as possible. Don't put it off. Most testicular lumps are NOT cancer, but early detection is vital if it is. This is a great guide for how to check yourself and perform a testicular self examination: http://www.cancerhelp.org.
I'd like every man reading this to go away an examine your balls, whether it's now, later on, or in the shower tomorrow. MAKE SURE YOU CHECK THEM!
And to any women who are in a relationship with a man, why not check his balls for/with him? It might be rare but it should never be missed!
I want you to check your balls because this year, when my partner (aged 24) was checking his, he found a lump that turned out to be testicular cancer. (It was caught early, treated quickly and he has been clear of cancer for over 6 months now.) I'm lucky to work at Lovehoney and have the opportunity to ask young men who are interested in their sexual well being to check their balls. Please do it, and please do it regularly.
If you are interested in any other information about testicular cancer, this is a great place to start: http://www.cancerhelp.org.