How different they can be? Most of the time when we talk about one we meant another.
Design is local and Architecture is global : When we are trying to design something it is very local in scope say interfaces for our classes while architecture is much broader like layering out the tiers and how they relate to each other. Architecture provides the blue print of the system.
Architecture is more about system behaviour : As opposed to design, architecture reflects business needs and quality attributes of the system, like the different ‘ilities’ such as modifybility, testability, upgradability, reliability, scalability, availability etc etc.
All said we should not try to come with a hard distinction between them, as although distinct concepts we have to deal with both of them to come up with a solution. Although not a good idea to have a big upfront design done but may be in some or the other we make some priliminary design decisions very early in the project which is very hard to change and is quite critical for the heath of the system. These early design decisions basically defines the architecture or may be is the architecture of the system.
