A collection of entity instances that share common attributes. If no candidate key exists or if the candidate key is a large composite key then choose a surrogate key. A owner can own several vehicles. A vehicle can have several owners.
Relationships operate both ways.
There can be more than one relationship between the same entities. Each employee of XYZ can complete several courses. The same course may be completed by several employees. XYZ needs to know the date on which an employee completed a course. Parent entity provides part of weak entity's primary key.