The vocabulary of The UML encompasses three kinds of building blocks: things, relationships and diagrams. Things in The UML are the abstractions that are “first-class” citizens in a model. Relationships tie these things together. Diagrams group interesting collections of things and their relationships; providing a projection of the model from a selected viewpoint.
There are four types of things in the UML: structural things, behavioural things, grouping things and annotational things.
Structural things are the nouns of UML models. These are the mostly static parts of a model, representing elements that are either conceptual or physical. Collectively, the structural things are called classifiers.
Behavioural things are the dynamic parts of UML models; the verbs of the model - representing behaviour over time and space. There are three primary kinds of behavioural things; interactions, state machines and activities.
Grouping things are the organisational parts of UML models. These are the “boxes” into which a model can be decomposed. There is one primary kind of grouping thing; packages.
Annotation things are the explanatory parts of UML models. These are the comments you may apply to describe, illuminate and remark about any element in a model.