UML

how objects are connected, how they react/communicate.
Visualize and design
UML used to visualize the design

• Associations simply show that there are some relationships between entities,static relation. ex: department offer courses.
• 0..* zero or more (on both sides "many to many" )
• 0..1 zero or one (one to one)
• 1..* one or more (one to many)
• Aggregation "has a" more specific than association "an association that represents (a whole part/ part) of a relation ship". ex: when a class is a collection/container of another class but the contained class doesn't have strong dependency on the container "if container destroyed, contents don't" (class container, students contained)
• Composition "Owns"  more specific than aggregation, a strong life cycle dependency between instance of the container class and the contained, if container is destroyed, normally every instance that it contains is destroyed (school container, classes contained).
• Generalization "Is a" (extends) solid.
• Realization "one model implements the behavior of the other" dashed.

i.e:

relationship between entities have to be abstract so that unnecessary details doesn't hide the relationships between them