There are two types of herpes, oral and genital. It's spread through skin-to-skin contact, usually during sex or kissing, and results in blisters lasting 7-10 days. Most carriers suffer outbreaks a few times a year, but that is controlable with proper meds. The odds of contracting the virus during an outbreak are are quite high, so if you kiss someone with a coldsore (coldsores are sympomatic of oral herpes), chances are you'll be infected. That said, genital herpes is usually painful and gross so it's unlikely you'd be having sex during an outbreak anyway. However, at any given time there is about a 5% chance of passing on the virus by way of sex, even if no sympoms are present (roughly 3 weeks/year). This is kind of scray as a lot of carriers never have sympoms, but can spread the virus.
Condoms are not 100% effective, but they never are anyway. No cure or vaccine exists either. Fortunetly, meds are pretty effective these days and a lot of people never even have outbreaks anymore.
(And yes, I had a "scare" a while ago which is why I know all this shit. Turned out to be nothing, fortunetly, but it made me a lot more careful.)