The word "carajo" is used mainly in the North of Spain, although any Spaniard will understand it, and its exact meaning as a noun is "dick". As far as I know, it can't be used as an adjective (at least not in Spain, maybe it can in some part of Latin America, but I don't think so); although you can say "del carajo" referring to persons or things: "cabrón del carajo".
The word "carallo", which is also the Galician word for "carajo", is also used. If you want a softer curse, the most used euphemisms are "caray" and "carainas", but many Spanish people will giggle if you use one of these.