In a Software Architecture diagram, you show the “skeleton” of your software, is like thinking in layers were you separate by layer the functionality and each layer is intercommunicates with the others surrounding them by priority or programming level.
In this phase of the Software Development Life Cycle the main concern is “what” a system should do while software Design is about “how” a system achieves its goals.
Architecture is a higher level of abstraction and it scopes all of the system, not just the modules like in the design.
Software Architecture focuses more on the interaction between the externally visible components of the system where as the Design is about how the internal components of the system interact with each other. Software Architecture is more about what we want the system to do and Software Design is about how we want to achieve that. Software Architecture is at a higher level of abstraction than the Software Design. Software Architecture is concerned with issues beyond the data structures and algorithms used in the system.