Hi Max,
this topic is discussed regularly here - a search on "association aggregation composition" should give you some usefull links.
It is also rather well explained in
http://www.objectmentor.com/UMLFJP [ February 26, 2003: Message edited by: Ilja Preuss ]
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