Answer:
A sales plan and a sales forecast are essentially the same, however the big difference is one is a set plan for the whole year. It is mostly used with larger corporations to single out units. A sales forecast is set at the store/branch level and is there to help with the changing situations (ie weather, natural disasters, and the such)
A plan is what you are going to do and a forecast is what you think is going to happen. There should be a link between the two, but you may not have the capacity to make all that you could sell.