From DifferenceBetween.com:
ER diagrams represent the abstract representation of the data model,
while class diagrams represent the static structure and behavior of
the proposed system. Main building blocks of ER diagrams are entities,
relationships and attributes but the main building blocks of class
diagrams are classes, relationships and attributes. Class diagram are
more likely to map in to real-world objects, while ER diagrams most
often map in to the tables in the database. Usually, relationships
found in ER diagrams are more difficult to understand for humans than
relationships in class diagrams.