One thing that I really like about UML is that there is a very clear distinction between the two categories. Whenever I'm going to build a diagram, I don't have to sift through every potential option out there, I can instantly say is this a structural type of system that I'm modeling or is it behavior-based. The names are relatively self-descriptive.

Structural Diagrams

Structural diagrams are any type of diagrams that model the way a system is architected. In other words, a class diagram is one of the most popular structural diagrams out there. It's a way of defining the different class names, attributes, and method names that belong inside of a system that you're building.

Behavioral Diagrams

Behavioral diagrams are much different, behavioral diagrams care about how your system behaves. It cares about different things like what type of messages does one system send to another or what types of permissions does user A have versus User B. Those are the types of questions that you're going to model out inside of behavioral systems.

So now that you know those two categories let's dive into each one of the diagrams that they fit into.