UML and SysML Diagrams -- an Overview

UML and SysML diagrams fall into three categories:   architectural, interaction, and behavioral.  Architectural diagrams show the structure of the elements of the model.  Systems Engineers using SysML use Block Definition Diagrams, Internal Block Diagrams to show a system's architecture.  Software engineers use class and object diagrams to show the software structure.  Both use package diagrams.  Though not discussed by here, Deployment Diagrams and Component Diagrams also show the the structure and full under the Architectural Diagram classification.

Interaction diagrams show how model elements communicate.  They include communication and sequence diagrams.  Of the two, I strongly prefer sequence diagrams because they are easier to understand.
Behavioral diagrams show what the model elements do when they receive communications.  Use Case diagrams, Activity diagrams, State Charts, and timing diagrams show the system's and the software's behavior.

Both UML and SysML are visual languages that use diagrams to show views of the model.  But remember that the diagrams are views, they are not the model.