Activity diagram is used to show identify the interaction
between domains and Sequence diagram is used to show the
logical execution flow of a business process.

Activity diagrams : 
are typically used for business process modeling, 
for modeling the logic captured by a single use case 
or usage scenario, or for modeling the detailed logic 
of a business rule. This is the OOAD version of flowchart 
or data flow diagram.

Sequence Diagrams:
model the flow of logic within your system in a visual manner, 
enabling you both to document and validate your logic, 
and are commonly used for both analysis and design purposes.
Sequence diagrams identify behavior within the system.