A forum is a lot more community based, with people chatting to each other being the main aspect. You have a lot to manage with a forum community wise. If someone doesnt like what another person says they can cause arguments so you have to manage them, you have to make sure people have soemthing to talk about, you have to make sure people do not spam...etc. A lot of it is community management.
You alos have to think about the layout of the board, the theme, where you are going to put the content, eg will you put an announcements section in? If so, will it go at the top of the forum or at the bottom? This makes a difference. The them is very important as it is something that can put someone off your forum.
As well as this there is the aspect you have to think about when running a normal site, content. It has to be unique, quality and get people to reply to it. A forum with no posts is useless. In conclusion, a fourm is like a website, but it also has teh added task of the community management aspect.