Genital herpes is caused by the herpes simplex virus (HSV) and is pretty much exactly the same as the virus that causes cold sores on the face. In 2004, there were 19,000 new cases diagnosed in the UK, with rates of infection in women 50% higher than rates of infection in men. It's so common that probably around 25% of sexually active people carry herpes, even though 3 out of 4 of these people won't show any herpes blisters. Because herpes can lie dormant, it's often difficult to know how or when you caught it.
The symptom, if it does occur, is a small blister than soon exposes a sore. Some people with herpes say that prior to a blister breaking out they feel a tingling on the exact spot where the blister will appear. The sore then crusts over and disappears. The timeframe from blister appearing to sore disappearing is around 10 days. Blisters may reappear at the same spot months or years later, although the frequency of recurrence tends to trail off over time.
Herpes is one of the STIs you can catch even if you always use a condom, though using condoms will certainly reduce your chance of catching it. The reason why a condom won't always protect you is that you catch herpes on the specific area of skin that rubs against the infected area on another person and a condom might not cover that specific part of your body - your groin, for instance. You can reduce your chances of catching genital herpes by checking that the other person doesn't have blisters, but you can catch herpes even if the blisters aren't there.
Typical ways to catch herpes are vaginal, anal and oral sex and you can catch it from someone who has either genital herpes or just cold sores. If someone with cold sores goes down on somebody else, that other person may well catch genital herpes. If someone who doesn't have herpes goes down on someone with genital herpes, the person going down could catch cold sores.
To test for genital herpes a doctor will normally take a swab from a blister or sore, though a doctor might be able to diagnose herpes just by looking at the infected area. If you're not showing any symptoms, a blood test is really the only way of telling whether you have been exposed to the infection but a blood test often isn't necessarily helpful: it gives no indication of the time of infection nor whether you will experience out-breaks in the future.
If you do have it, the symptoms can be managed but there isn't a cure. Because it's passed on to other people very easily, you should manage your sex life accordingly.