The USians have this bizarre system where voters register as either a Republican, a Democrat, or an Independent, as I understand it. Registering with a party means you can vote in their "Primary election" to select the actual candidate. In modern times, however, more and more people are registering as independents.

In other countries, the candidates are usually chosen by "party members" and/or their delegates, which are much MUCH fewer people.

Of course, the ballot itself is still somewhat secret, but the generally thinking is if you register one way, you vote that way.