Caching vs. Replication in DNS

[Andersen]

Hi,

I don't really understand the relationship between caching and 
replication in DNS. Isn't replication used very little, you only have a 
master and slave server typically for each zone?  And the root servers 
are 16 replicas?

What I really want to understand is where caching is being used. It is 
said that it takes time before updates are propagated due to caching, 
where does this apply? If I just want to change the IP address of an 
existing hostname, shouldn't an update to the corresponding nameserver 
immediately make the change visible around the globe. Or how long would 
it take?

What if I want to insert another level, for instance I have d.c.b.a 
running, I want to create e.d.c.b.a? That would be reflected immediately 
right?

regards,
Andersen