According to me, architectural design is the design process of a single site, be it a school, a single dwelling, a commercial complex or a resedential township. But it is strictly a single large or small site, the proportions of which is restricted by local govt body.
Urban Designing refers to a greater context, a much larger area having a huge number of the above ‘sites’ and a much larger human population. Again, the proportion of land coming under the jurisdiction of an Urban Designer is also fixed by the local govt body, but it is usually much larger, containing several thousands of smaller individual units.
Where the architect deals with a single dwelling and the various plumbing and electrical issues associated with it, the urban designer takes care of the sanitary disposal pattern, public gathering spaces and such issues associated with the whole community within that urban area. They have different operational scales, deal with different problems and have different objectives associated to them. So i think they are more or less different.