Object Relationship Definitions
A nice overview of relationships types...
Link: A 0:0 optional relationship basically states that a person can occupy one parking space, that I don't need a person to have a space and I don't need a space to have a person.
SubType: A 1:0 relationship; optional only on one side. This would indicate that a person might be a programmer, but a programmer must be a person. It is assumed that the mandatory side of the relationship is the dominant.
Physical Segment: A 1:1 mandatory relationship and demonstrates a segmentation denormalization. A person must have one and only one DNA pattern and that pattern must apply to one and only one person.
Possession: A 0:N (zero to many) optional relationship indicating that a person might have no phone, one phone or lots of phones, and that a phone might be un-owned, but can only be owned by a maximum of one person.
Child: A 1:N mandatory relationship, the most common one seen in databases. A person might be a member or might not, but could be found multiple times (if the member entity represents membership in multiple clubs, for instance).
Characteristic: A 0:N relationship that is mandatory on the many side. It indicates that a person must have at least one name, but possibly many names, and that a name might be assigned to a person (might not) but at most to one person.
Paradox: A 1:N relationship mandatory on both sides. The "Chicken and the Egg" is involved since you have to have a person to have citizenship, but citizenship to have a person.
Association: A N:N (many to many) optional relationship. Conceptually, it means that a person might or might not work for an employer, but could certainly moonlight for multiple companies. An employer might have no employees, but could have any number of them. Again, not hard to visualize, but hard to implement. Most solutions of this situation involve creating a third "Associative Entity" to resolve the M:M into two 0:M relationships. This might be an entity called employee because it does link the person to the employer the person works for.