The soul is dyed the color of its thoughts. Think only on those things that are in line with your principles and can bear the light of day. The content of your character is your choice. Day by day, what you do is who you become. Your integrity is your destiny - it is the light that guides your way. - Heraclitus
Here's how I address that topic in BJO (paraphrasing):
An association is a relationship between two classes A and B, implemented in code as follows:
one-to-one -- we provide A with a reference to an object of type B as an attribute (and vice versa, if we want the association to be bi-directional)
one-to-many -- if there is one instance of A for many instances of B, we equip A with a Collection of references to B as an attribute
many-to-many -- both A and B wind up with a Collection of the other as an attribute (if bi-directional)
Aggregation -- essentially an association that implies containment -- e.g., "has a", "is part of", "belongs to", etc. Aggregations are represented in code identically to associations, as discussed above.
Composition -- a strong form of aggregation in which the "contained" object ceases to exist if the "containing" object ceases to exist. Again, coded in the same way that associations/aggregations are coded, but extra work must be done programmatically to make sure that the contained object is "snuffed" when the containing object is.