UML class diagrams are largely composed of boxes - basically a rectangular border containing text. UML class diagrams use boxes to represent units of code - i.e. classes, structs, and enumerations. These boxes are broken into compartments. For example, an Enum is broken into two compartments:

Stereotypes

UML is intended to be language-agnostic. But we often find ourselves in situations where we want to convey language-specific ideas, and the UML specification leaves room for this with stereotypes. Stereotypes consist of text enclosed in double less than and greater than symbols. In the example above, we indicate the box represents an enumeration with the $ \texttt{<<enum>>}$ stereotype.