Please explain identifying/non-identifying relationships

[Chris Bassett]

Can someone please explain to me the need for identying relationships and also how they differ from non-identifying?

I'm aware that they you create an identifying relationship, the FK of the other table becomes part of the PK of the current table (so it would then have a multi-column PK). But what does this do? Does this ensure that the "parent" element exists before the "child" element can exist?

Does this enforce integrity?

I'm building a product matrix that has various levels (sort of like a system where you can "Drill down" on a particular product from generic to specific) and have a bad feeling that this issue may be important, and don't want to find out that I did it wrong later on...