[dba-SQLServer] Relation integrity constraints

I have used database diagrams to create relationships between FK / PK pairs in tables such as client 
/ product etc.  My understanding is that such relationships create constraints, IOW enforce 
relational integrity by ensuring that parents cannot be deleted while child records exist and so forth.

First of all, is this true?

Second, where are these constraints stored?  I can see constraints in the table itself, but these 
seem to be "value" kind of constraints, i.e. Disallow zero length and the like.  Where is the 
"parent / child relationship info stored?

Thanks,

-- 
John W. Colby
Colby Consulting

Reality is what refuses to go away
when you do not believe in it